vtc_haproxy: enable mcli with a separate flag from worker mode

    Support for the master cli in worker mode was added with commit 86e65f1
    using the same flag as worker mode (-W). This is a problem with haproxy
    1.8 because it has worker mode but no mcli. This patch adds -S to enable
    mcli for haproxy in a vtc.

and:

    The two patches in attachment fix a bug in the haproxy CLI handling and add
    the support for the haproxy master CLI in VTest. Could you merge them? Thanks
    You can also find in attachment a vtc file which use this new feature.

    Submitted by:   William Lallemand <wlallemand@haproxy.com>