For the most part by adding go docs, and by using Go naming
conventions in a few cases.

Remove some commented-out code while we're here.

Quiets golint.

Use the label key viking.uplex.de/secret. The controller only reads
Secrets with this label, and with the field type:kubernetes.io/tls
(the latter are Secrets specified for Ingress).

Three values are permitted for the label:

admin: credentials for remote admin of Varnish and haproxy (Varnish
shared secret and Basic Auth password for the dataplane API).

pem: initially empty Secret into which the controller writes pem
files (concatenated crt and key), projected into a volume from
which haproxy reads at load time. Currently only with the hard-
wired name "tls-cert", so that RBAC update privileges can be
limited to this Secret.

auth: credentials for Basic and Proxy Auth, as configured via
the VarnishConfig custom resource.

We read Secrets with labels that identify a Secret for use by this
application. These include:

- Secrets for the remote administration of Varnish and haproxy
  (to authorize use of the Varnish CLI and the dataplane API for
  haproxy).

- Secrets for applications like Basic and Proxy Auth.

- The Secret in which PEM files for haproxy are created, and
  is projected into a volume that haproxy reads. This is how we
  create TLS material for use by haproxy (which requires that
  crt and key are concatenated into one file).

We also read Secrets with the type field set to "kubernetes.io/tls".
These contain the TLS material, and are the Secrets named in an
Ingress spec.

This has necessitated adding two new informers to the controller,
for which the filters are defined.

We take advantage of the fact that if a directory is specified for
haproxy as the location for certificates, then haproxy reads all
of the certificates found there, and automatically associates the
SNI of incoming connections with the appropriate certificate.

It contained a newline, which rendered the sed command in the entry
point of the haproxy container invalid.

The entrypoint of the haproxy container is an exec
script that sets the password from a template for
haproxy.cfg. Setting the password from an env
variable has proven to be too unreliable.

Also, listening at a UDS does not appear to be
working at all. So the dataplane API listens directly
at the container port (no intervening haproxy
frontend).

This change makes the container more friendly to
read-only filesystems, since haproxy.cfg is now no
longer modified in /etc, but rather in the ephemeral
file system at /run (== /var/run).

Most importantly, this tells us the current version number in use
by the dataplane API, which apparently insists that the number is
always counted up (otherwise there are 409 Conflict responses).

The Secret must be in the same namespace of the Pods into which
their contents are mounted.

The verification script intermittently gets a 503 status if requests
are sent to "soon", even after waiting for the Varnish Services to
become ready. It doesn't appear to happen if we wait a few seconds
longer.

For now, wait longer until we run the verification test case. In the
long run, we should investigate why the configuration is not actually
ready when the Ready state is reached.

Since the controller now interacts with the headless Service that
defines the admin ports, it can no longer find the http port in
that Service definition. This is needed to configure the Varnishen
as backends for one another.

Search for all Services in the same namespace that define the same
selector as the admin Service (and hence are configured for the
same Pods). We then search for the http port in the Endpoints of
those Services.