rbd/librbd: fix the cache mode facet

This appears to have been broken, which means we weren't actually testing
any of the non-default cache modes (writethrough, none).  Whoops!  We were
getting jobs like

task:
- install: null
- ceph:
    conf:
      client:
        rbd cache: false
tasks:
- chef: null
- clock: null
- install: null
- ceph: null
- rbd_fsx:
    clients:
    - client.0
    ops: 5000

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit e747abef4ee54f2011616fef7d32efa10e0935e1)

Conflicts:

	suites/rbd/librbd/cache/none.yaml
	suites/rbd/librbd/cache/writeback.yaml
	suites/rbd/librbd/cache/writethrough.yaml
	suites/rbd/librbd/ceph/ceph.yaml
clusters: add mds to fixed-1.yaml

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 425f5dd8615a2603e46f70e392a9586d07c15db5)