Allow customization of RDO release to use on RHEL7

By removing rdo-release-icehouse from the undercloud source JSON
we can customize which version of RDO to use exporting RDO_RELEASE.

The cleanup script is also updated to remove any version of the
rdo-release package found.
Ensure we use rdo-icehouse if RDO_RELEASE is left unset
Merge pull request #35 from gfidente/rdo_release

Allow customization of RDO release to use on RHEL7