contint: ensure /mnt is mounted on eqiad

We publish a bunch of resources under /mnt which on pmtpa is a dedicated
partition (/dev/vdb).  On eqiad /mnt is on the main disk and we use
labs_lvm to create an additional disk.

When deploying Jenkins slaves in eqiad, the resources under /mnt ended
up being released before /mnt got mounted.

The crazy hack is to define the requirement of files under /mnt to be
File['/mnt'] on pmtpa but Mount['/mnt'] on eqiad.

Change-Id: I14fad16e6654ef6d74e25e96861c6b3538ad7f0d
Merge "contint: ensure /mnt is mounted on eqiad" into production