labs_vagrant: Ensure that lvm volume is mounted first

Tell puppet to make sure to mount the LVM managed disk before applying
Labs_vagrant. There is still a possible issue with this hiding content
that existed on the /srv mount point before the LVM volume is added, but
that seems like an issue that may be better solved in Labs_lvm::Volume
itself.

Change-Id: I8cb5f94fcacd3498823bea1e2de342f2e6a4548b