mgr: initialize PyModuleRegistry sooner

mgr: initialize PyModuleRegistry sooner

This was waiting on MgrMap to init, so that it would know whether
modules should be enabled.  However, you don't really need to know
that until someone calls standby_start or active_start, so we
can initialize during MgrStandby::init, and fill out the enabled
flags later.

This prevents the mgr from prematurely emitting a MMgrBeacon
that claims no modules are found.

http://tracker.ceph.com/issues/22918
Signed-off-by: John Spray <john.spray@redhat.com>