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>