mon: AuthMonitor: validate caps when creating or changing mon caps

mon: AuthMonitor: validate caps when creating or changing mon caps

The monitor doesn't really know how to validate caps not meant for it.
The MDS or the OSD may very well allow blank caps for instance, while
the monitor categorically does not.  We can't simply state a capability
is invalid because we wouldn't take it as such.

On the other hand, we must check monitor caps and make sure they are
correct, otherwise malformed caps can go unnoticed for a while,
sometimes even being hard to understand what may have gone wrong.

Backport: firefly

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>