mon/OSDMonitor: 'osd tier cache-mode <pool> <mode>'

Signed-off-by: Sage Weil <sage@inktank.com>
Signed-off-by: Greg Farnum <greg@inktank.com>
qa/workunits/cephtool/test.sh: test osd tier CLI

Signed-off-by: Sage Weil <sage@inktank.com>
mon/OSDMonitor: 'osd tier {set,remove}-overlay <pool> [tierpool]'

Also prevent 'osd tier remove ...' if the tierpool is the current overlay.

Signed-off-by: Sage Weil <sage@inktank.com>