Read template's expected value orchest test Part-1

In tests under /tempest/api/orchestration/, resources gets
created based on parameters defined in template (yaml, json).
But while verification of output, expected values are hard coded
in tests. Expected values should be read from template only.

This patch adds the helper to load the values from template
(yaml, json) and apply the changes in
tempest/api/orchestration/test_volumes.py

This patch also rename the existing load_template() function
as it read the template and newly added function load the template
This rename the load_template() to read_template in
tempest/api/orchestration/base.py and corresponding calling files.

Partial-Bug: #1327919

Change-Id: Ie0817e20cb7bacdf33b16fb222a7ee9c91109fc8
Merge "Read template's expected value orchest test Part-1"