[RFE] Periodic tasks: Allow passing a function callback to the spacing parameter

[Lucas Alvares Gomes]

We could allow passing a function callback to the "spacing" parameter of the @periodic decorator, that would make it more flexible and would enable things like refreshing the interval values when the service re-loads the configuration file (in case of a SIGHUP). For example:

 def _sync_states():
    return CONF.sync_states_interval

 class Foo(object):

     @periodics.periodic(spacing=_sync_states)
     def sync_states(self):
         ...

Alternatively we could have another parameter for that as well, "spacing_func" maybe?