mgr: transmit command descriptions to mgr in activating beacon

mgr: transmit command descriptions to mgr in activating beacon

The mgr already sends a beacon to the mon immediately
after loading python modules in Mgr::init, to indicate
that it is now available.  Use that beacon to transmit
the command descriptions.

The monitor should handle this beacon by persisting
the command descriptions before persisting the updated
mgrmap that indicates that the mgr is now active.

Signed-off-by: John Spray <john.spray@redhat.com>