OSDMonitor: do not add non-empty tier pool unless forced

In general, users should not use non-empty pools as new tiers or else
things can behave strangely:

 - the data sets are unrelated behavior will be... strange.
 - if the cache pool is not "new" and does not do the OMAP flag, the OSD
   will not know not to flush omap objects to an EC base tier
 - probably other random stuff I'm forgetting

Allow a user to shoot themselves in the foot with --force-nonempty.

Implements: #7457
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1335 from ceph/wip-tier-add

mon: prevent non-empty pools from being added as tiers

Reviewed-by: Greg Farnum <greg@inktank.com>