Correct lock path for storage-registry-lock

This fixes the lock path for storage-registry-lock to prevent a
potential data loss due to a race-condition scenario.  Since the
storage-registry-lock is used to modify a json blob that is
authoritative as to which nodes are using the storage so that
we don't remove any "in-use" backing files on the shared storage
segment (if in use).

The potential data-loss could occur in limited cases of the race
that the lock should prevent.  This can only occur if the image
cache cleanup is triggerd when a node has been eliminated from
the register_storage_use JSON incorrectly by this race condition.

Most of the time the race simplyresults in a corrupted json blob.

Fixes-bug: 1194696
Change-Id: Ib1a5785646e75dc90bdd2a7e69a697adc4fbb298
Merge "Correct lock path for storage-registry-lock"