mon/OSDMonitor: prevent setting hit_set unless all OSDs support it

We are using OSD_CACHEPOOL as a proxy for the support for the tiering
OSDMap infrastructure.

Signed-off-by: Sage Weil <sage@inktank.com>
mon/OSDMonitor: require OSD_CACHEPOOL feature before using tiering features

The OSDs need to support this feature before we allow users to turn it
on.  This is similar to what the erasure pool support does.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge branch 'wip-7875'

Reviewed-by: Samuel Just <sam.just@inktank.com>