Change DB API instance functions for selective metadata fetching

This makes the DB API for instance_get_all() variants able to take a
columns_to_join parameter like the base instance_get_all() function,
which provides control over whether we go to the trouble of pulling
the metadata out of the database before we return the instances.

Related to bug 1164737

Change-Id: Ie7c012e2e12bb91ef4294cdc90d828af255b67c5
Optimize some of the periodic task database queries in n-cpu

There are a few trivial periodic tasks that can benefit from telling
the DB API (via conductor) not to fetch metadata items. This patch
makes those changes

Related to bug 1164737

Change-Id: I8e2730d3ed74bd7e0a0217647316c1cf492f22f3
Merge "Change DB API instance functions for selective metadata fetching"
Merge "Optimize some of the periodic task database queries in n-cpu"