Drop role::zuul::labs

That was a back compatibility role class which is no more needed.
Instead we use on labs:

    include role::zuul::merger
    include role::zuul::server

Both classes use a configuration hash to vary settings between
production and labs.

role::zuul::production can probably be dropped as well later on.

Change-Id: I016dbec8f08d71a6b52ebfbf6ec0b9af53a258c2
Merge "Drop role::zuul::labs" into production