mgr: create always_on class of modules

mgr: create always_on class of modules

support an 'always on' class of ceph-mgr modules. the purpose of an
always-on module is to behave as built-in / non-optional functionality.
this is accomplished by treating always-on modules as enabled
irregardless of the enabled set produced by the manager monitor.
always-on modules that fail to load are reported as health errors.

always-on modules are intended to satisfy important dependencies in a
cluster. as such, the set of always-on modules is specified statically,
as opposed to a property on the Python class, to enable robust error
reporting of always on modules that fail to load.

Signed-off-by: Noah Watkins <nwatkins@redhat.com>