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.

Change-Id: I84405d1d2f5245ab7cb8b909c2ea1407d5b2973e
Closes-Bug: #1388985
(cherry picked from commit 09bc7ad79e4d95cffd8db19c781dc66230640d84)
Merge "Use environment file in template-validate" into stable/juno