Change nova scheduler filters

This change adds DiskFilter and changes AggregateCoreFilter to
CoreFilter.  In stable/juno we have AggregateCoreFilter and
AggregateDiskFilter, but AggregateDiskFilter is not available in
icehouse.  If someone actually needs the aggregate versions of
these filters then they can set local overrides.

Change-Id: I1f41ad386460f71629205798db06fbad6ccbfe9b
Closes-Bug: 1399364
Merge "Change nova scheduler filters" into stable/icehouse