mon: OSDMonitor: return immediately if 'osd tier cache-mode' is a no-op

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: OSDMonitor: disallow nonsensical cache-mode transitions

Fixes: 8155

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Merge pull request #1740 from ceph/wip-8155

mon: OSDMonitor: disallow nonsensical cache-mode transitions

Reviewed-by: Sage Weil <sage@inktank.com>