Glance auth_version needs to be in conf by default

When auth_version value is v2.0, it still should explicitly
set auth_version to v2.0 as the default value in configuration file.
This is also to be consistent with other components. The current
code does not have 'auth_version = v2.0' in the configuration file
if the value is v2.0.

Change-Id: Ic39626f2d40d8ca6f5d5fc5864e5b5407b71d730
Closes-Bug: #1387067