Improve rpm package build requirements for anvil

Currently it is a major pain to build horizon, because to correctly build horizon you need to have all the openstack clients installed on the build host as well as some other openstack specific packages. We use to do a hack that would remove the requirements of installing all the clients in order to get python manage.py compressstatic to run. Under icehouse there is a new requirement to install a package that are not available in standard repos - unless you install/enable RDO, or have access to a previously built set of openstack packages. The current specific package is python-oslo-config,which is built in the normal package phase and not in the dep phase and as such is not available for install at horizon build time.

A possible fix to this would be to add a mini-stage before prepare/build that is called prepare-build-requirement and build-build-requirements so that the needed deps are built and placed into a repo before they are included/used.