Refactor periodic tasks.

This review allows periodic tasks to be enabled or disabled in the
decorator, as well as by specifying an interval which is negative.

The spacing between runs of a periodic task is now specified in
seconds, with zero meaning the default spacing which is currently 60
seconds.

There is also a new argument to the decorator which indicates if a
periodic task _needs_ to be run in the nova-compute process. There is
also a flag (run_external_periodic_tasks) which can be used to move
these periodic tasks out of the nova-compute process.

I also remove the periodic_interval flag to services, as the interval
between runs is now dynamic based on the number of seconds that a
periodic task wants to wait for its next run. For callers who want to
twiddle the sleep period (for example unit tests), there is a
create() argument periodic_interval_max which lets the period
periodic_tasks() specifies be overridden. This is not exposed as a
flag because I cannot see a use case for that. It is needed for unit
testing however.

DocImpact. Resolves bug 939087.

Change-Id: I7f245a88b8d229a481c1b65a4c0f1e2769bf3901
Merge "Refactor periodic tasks."