labs_vagrant: Install to /srv/vagrant

In pmtpa labs, instances had a large secondary disk mounted on /mnt.
Yuvi had the labs_vagrant class install to this directory due to the
potentially large size of a working MediaWiki-Vagrant checkout. In
eqiad, secondary disks must be mounted explicitly. Update
role::labs::vagrant to require role::labs::lvm::srv which makes a large
secondary disk available at /srv. Change ::labs_vagrant to install to
/srv/vagrant rather than /mnt/vagrant. ::labs_vagrant includes new logic
to migrate existing /mnt/vagrant deployments to the new /srv/vagrant
location transparently.

Change-Id: I06c1e5a537bbf923ac5a7b0a787f4e9152bd9045
Merge "labs_vagrant: Install to /srv/vagrant" into production