Refactor nagios3 element to refresh more often

Previously the element would only refresh from Nova when the
Heat configuration was changed. But we want it to refresh
whenever nova changes.

Note that we won't actually remove hosts that go away.

Change-Id: Ic8af7f30ccc0cc2c64643081a6e300a354d8ad35
Merge "Refactor nagios3 element to refresh more often"