contint: use role::labs::lvm::mnt on eqiad slaves

Andrew Bogott introduced a role class to allocate the eqiad instance
disk and mount it at /mnt.  Use that role instead of a local hack.

Reference:
8e459cfd / https://gerrit.wikimedia.org/r/#/c/119398/

Change-Id: Iafb30f0c2dcde471d2e8f917da3cf94630aba360
Merge "contint: use role::labs::lvm::mnt on eqiad slaves" into production