Remove warning & change @periodic_task behaviour

During development of Juno, Ia227f4c4e69ecf361ab02d1d17a3010303650104
added a warning that the behaviour of periodic_tasks with an interval of
0 would change.  This patch changes the behaviour and removes the
warning.

Oslo (which provides the @periodic_task decorator) uses spacing=0 to
mean "run at the default interval of 60s", but several nova methods used
spacing=0 to mean "don't run".  These methods now behave consistently
with the Oslo code.

Due to an error in the original patch, some of the warnings refer to the
intended behaviour change as being in "the K release", and some to "Juno".
The warning about changing behaviour is first present in Juno, so this
patch should be merged in Kilo.

Change-Id: Ie5f2092a3b84e3674fe6a68ce511d2dc218bb625
Closes-bug: 1276203
Merge "Remove warning & change @periodic_task behaviour"