Previously all sync operations (add/update/delete for the resource
types that the controller watches, and the cluster changes brought
about for them if necessary) were only characterized by a Go error
variable. The only conditions that mattered were nil or not-nil.

On a nil return, success was logged, and a SyncSuccess event was
generated for the resource that was synced. But this was done even
if no change was required in the cluster, and the resource had
nothing to do with Ingress or the viking application. This led
to many superfluous Events.

On a non-nil return, a warning Event was generated the sync
operation was re-queued, using the workqueue's rate-limiting delay.
This was done regardless of the type of error. Since the initial delay
is quite rapid (subsequent re-queues begin to back off the delay), it
led to many re-queues, and many Events. But it is very common that a
brief delay is predictable, when not all necessary information is
available to the controller, so that the rapid retries just
generated a lot of noise. In other cases, retries will not improve
the situation -- an invalid config, for example, will still be
invalid on the next attempt.

This commit introduces pkg/update and the type Status, which classifies
the result of a sync operation. All of the sync methods now return
an object of this type, which in turn determines how the controller
handles errors, logs results, and generates Events. The Status types
are:

Success: a cluster change was necessary and was executed successfully.
The result is logged, and an Event with Reason SyncSuccess is generated
(as before).

Noop: no cluster change was necessary. The result is logged, but no
Event is generated. This reduces Event generation considerably.

Fatal: an unrecoverable error (retries won't help). The result is
logged, a SyncFatalError warning Event is generated, but no retries
are attempted.

Recoverable: an error that might do better on retry. The result is
logged, a SyncRecoverableError is generated, and the operation is
re-queued with the rate-limiting delay (as before).

Incomplete: a cluster change is necessary, but some information is
missing. The result is logged, a SyncIncomplete warning Event is
generated, and the operation is re-queued with a delay. The delay
is currently hard-wired to 5s, but will be made configurable.

We'll probably tweak some of the decisions about which status types
are chosen for which results. But this has already improved the
controller's error handling, and has considerably reduced its
verbosity, with respect to both logging and event generation.

These correspond to properties that can be set in VMOD dynamic.
Currently we have:

dnsRetryDelay: this is set as the ttl in the VMOD. Since we get
DNS TTLs from the server, it effectively sets the retry delay
after a lookup gets negative results. More recent versions of the
VMOD have a separate parameter for this purpose, so this should be
updated soon.

domainUsageTimeout: corresponds to the domain_usage_timeout param
of the VMOD director.

firstLookupTimeout: corresponds to the first_lookup_timeout param
of the VMOD director.

resolverTimeout: set with the .set_timeout() method of the VMOD
resolver object.

resolverIdleTimeout: set with the .set_idle_timeout() method of the
VMOD resolver object.

maxDNSQueries: set with the .set_limit_outstanding_queries() method
of the VMOD resolver object.

followDNSRedirects: set with the .set_follow_redirects() method of
the VMOD resolver object.

Ref gitlab issue #20

Ref gitlab issue #20

Uses VMOD dynamic, and requires that the getdns library is installed
in the image running Varnish. This allows us to use dynamic.resolve(),
in particular so that TTLs from DNS are honored.

Currently sets ttl to a hard-wired value of 30s. Since the TTLs for
lookup are obtained from DNS, this actually sets the delay until
lookups are retried after negative results (default 1h).

The next step is to test and extend BackendConfig support to configure
properties of VMOD dynamic. That will make it possible to configure
the ttl value (although we might stay with a much shorter ttl than
1h).

Partially addresses gitlab issue #20.

These weren't necessarily being changed after an Endpoints update.

The Ingress update may have followed an update for Endpoints.

XXX: re-use this code wherever we wait for port-forward.

For that, the Secret must be named as the TLS Secret by an Ingress
in the same namespace that identifies out ingress.class.

This means that the controller doesn't need to try delete an element
from any PEM Secret (to remove the certificate from the haproxy
Secret volume).