With this change, [compute]resource_provider_association_refresh is
allowed to be zero, which will disable refreshing resource provider
traits and aggregates.
Inventories are still refreshed in a different code path.
A subsequent patch will be submitted to allow manual refresh by sending
SIGHUP to the compute process.
Change-Id: Iec33e656491848b26686fbf6fb5db4a4c94b9ea8