Make heat depend on python-heatclient

This will be required soon for one or more of the
following reasons:
* Multi-region invoking stacks on other heat endpoints
* A variant of nested stack which goes through API instead of
  internally
* Orchestrating new heat-based resources (autoscaling, software config)
* Possibly sharing some template parsing and validation between
  client and server by moving this code to client

Change-Id: Ia3e55d1b3bb19e713e4c1ecc341805f3a208844e

requirements.txt

@@ -16,6 +16,7 @@ requests>=1.1
 Routes>=1.12.3
 SQLAlchemy>=0.7.8,<=0.7.99
 WebOb>=1.2.3,<1.3
+python-heatclient>=0.2.3
 python-keystoneclient>=0.4.1
 python-swiftclient>=1.5
 python-neutronclient>=2.3.0,<3