Allow $OS_AUTH_URL environment variable to override config file value

Make the Glance APIs use this value in a consistent/expected manner
when it is set.  The Principle of Least Surprise dictates that
setting the value in the environment should override any values in
the config files.

Change-Id: Icdc39a2fd0da71a75f4ad0e60e34cfb8e1883787
Closes-Bug: 1314354
Merge "Allow $OS_AUTH_URL environment variable to override config file value"