mon: implement safety interlock for deleting pools

This is a very easy way for users to accidentally to a *lot* of damage.
Make it an annoying manual process to actually do this.

Signed-off-by: Sage Weil <sage@inktank.com>
rados: safety interlock on 'rmpool' command

This is a very easy way for a user to do a lot of damage with no way back.
Make sure they mean it.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge remote-tracking branch 'gh/wip-rm-pool'

Reviewed-by: Samuel Just <sam.just@inktank.com>
PendingReleaseNotes: pool removal cli changes

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