mon,mgr: manage mgr module list in mgrmap

mon,mgr: manage mgr module list in mgrmap

Kill old mgr_modules option.

Add new mgr_initial_modules option, on the mon, for the initial cluster
mgrmap.

Add ls, enable, disable commands.

Respawn mgr if the module list changes.  In the future we could enable
new modules without a full restart, but disabling probably requires (and
is best handled by) a respawn.

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