Ephemeral Heat for the Overcloud

This blueprint proposes using the ephemeral Heat stack model for all deployments
types, including the overcloud. Using ephemeral Heat is already done for
standalone deployments with the "tripleo deploy" command, and for the
undercloud install as well. Expanding its use to overcloud deployments will
align the different deployment methods into just a single method. It will also
make the installation process more stateless and with better predictability
since there is no Heat stack to get corrupted or possibly have bad state or
configuration.