Use TripleO Heat Merge to manage the stack

Generate the YAML file describing Tuskar compute nodes and invoke Heat
Merge. This review adds the master branch of tripleo-heat-templates as
a requirement.

At the moment this patch hardcodes the parameters which define the
different Overcloud Roles. On a create over the Overcloud it creates one
controller and one compute Overcloud role. When the Overcloud is updated
it created a second compute node.

This can be manually tested with these commands.

Create the overcloud:
curl -H "Content-Type:application/json"\
 -XPOST http://localhost:8585/v1/overclouds/ -d '{}'

Update the overcloud:
curl -H "Content-Type:application/json"\
 -XPUT http://localhost:8585/v1/overclouds/1 -d '{}'

Delete the overcloud:
curl -H "Content-Type:application/json"\
 -XDELETE http://localhost:8585/v1/overclouds/1 -d '{}'

Change-Id: I578b4e9f238590ea245b827bc75d252568d194fe