mon/MonCommands: 'invalidate+forward' -> 'forward'

Commit 4e439857a694 introduced invalidate+forward cache mode, commit
81279e3bb6e0 renamed it to forward, but missed the CLI.  Fix it.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #1124 from ceph/wip-cache-mode-fix

mon/MonCommands: 'invalidate+forward' -> 'forward'

Reviewed-by: Sage Weil <sage@inktank.com>