Allow user to specify mount point for role::labs::lvm::mnt

Check for a global variable named `lvm_mount_point` to change the
default mount point from /mnt.

Once this is merged, the variable should be added to the global
labsdrives group on wikitech so instances can use it.

Change-Id: I9c949788f99b2e524086852c3c3201a64dfc2695
Merge "Allow user to specify mount point for role::labs::lvm::mnt" into production