Add warning to periodic_task with interval 0

The behaviour of Oslo's @periodic_task decorator is that
a repeat interval of 0 means that the method will be
called regularly.  Several Nova methods which use that
decorator also have code for returning early (ie no-op)
if the interval is 0.  This is inconsistent for users, and
means there is timing-related code where it doesn't belong.

This patch adds a warning when a user takes a value of 0
from config and uses it as an interval in a @periodic_task.

Ideally all code will use Oslo's convention that 0 means
run at the default rate and a negative interval will mean
"don't run".  After this warning has been in place for a full
release cycle that can be the case.

Change-Id: Ia227f4c4e69ecf361ab02d1d17a3010303650104
DocImpact: Warns of upcoming change to behaviour when
           *_interval is set to 0
Closes-Bug: #1272830
Partial-Bug: #1276203
Merge "Add warning to periodic_task with interval 0"