Default to xenserver driver if xenserver-core is installed

Fixes bug 1209205

Change-Id: I63085cc87610a59fc48e519e4351c9233b3961be
Support heat in standalone mode.

The following localrc will launch only heat in standalone mode
and allow it to provision within the openstack specified by
the configured keystone endpoint:
HEAT_STANDALONE=True
ENABLED_SERVICES=rabbit,mysql,heat,h-api,h-api-cfn,h-api-cw,h-eng
KEYSTONE_SERVICE_HOST=...
KEYSTONE_AUTH_HOST=...

Change-Id: I0d8a541fc9d592577423b074c789829f8b8d6702
Merge "Default to xenserver driver if xenserver-core is installed"
Merge "Support heat in standalone mode."
Merge branch 'master' of github.com:openstack-dev/devstack