Use environment file in template-validate

Use the -e parameter in template-validate to override any
Default values specified in the template.  This has the desirable
effect of making Horizon use parameters updated from the environment
file.

Conflicts:
	heat/engine/service.py
	heat/tests/test_validate.py

Change-Id: I84405d1d2f5245ab7cb8b909c2ea1407d5b2973e
Closes-Bug: #1388985
(cherry picked from commit 09bc7ad79e4d95cffd8db19c781dc66230640d84)
(cherry picked from commit dc1b99d064d9fbb6439357e89c770f15810f7cce)