Support scoped keys in aggregate extra specs filter

Update AggregateInstanceExtraSpecs to support scoped keys in flavor
extra_specs.  Otherwise, you can't use this filter in combination with
other filters that act on un-scoped extra specs, because they may
conflict.

This recently came up on the mailing list:

http://lists.openstack.org/pipermail/openstack-dev/2013-July/011421.html

Fix bug 1198290.

DocImpact - See updates to filter_scheduler.rst.

Change-Id: I03d1d3268c800dc6982ffa4b13f8b9489428b991
Merge "Support scoped keys in aggregate extra specs filter"