Use packaged heatclient in the heat-config element

Updates the heat-config element so that it optionally supports
installing heatclient via a package instead of pip.

To do this you would set DIB_INSTALLTYPE_heat_config=package
in your environment before running DIB.

Ideally TripleO would have split out elements for each
client library thus allowing us to add something like
'heatclient' to our element-deps.

We don't have that yet... today there is only openstack-clients which
installs *everything*. So perhaps we can take this approach for
now and then migrate to using something cleaner if/when it exists.

Change-Id: Ic5fd5efef0034cff2084d81f2c4cd5adc083b4b4