deployment: abstract out MW_RSYNC_HOST

Hack up the deployment var class to let beta uses a different MediaWiki
synchronization host.  The 'if $::realm' is not clever, but I am not
really willing to refactor the messy deployment manifests.

Change-Id: I3eb5b5c4e7f255c83758d96f5999980c5cf7f31b
Merge "deployment: abstract out MW_RSYNC_HOST" into production