mgr: execute modules even if can_run=false

mgr: execute modules even if can_run=false

We execute modules even if can_run=false, so that it is possible
to load them for running their selftest hooks.  However,
we already raise health messages about the fact that they're
enabled but can't run, so we don't want to also raise
health messages about whatever exceptions they raise
from serve()

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