The manila cephfs-nfs driver jenkins gate has been failing for a while
since it started using Ceph >= 12.1.1. The manila driver is unable to
fetch Ceph cluster usage stats as it did with earlier Ceph releases.
Temporarily revert to using Kraken 11.2.x stable packages to unblock
the failing gate.
Partial-Bug: #1708169
Change-Id: I4e842e2cee14755e02714e856114a5eabb5613b3