Register used CONF entries in cinder.api.common.py

After change: I2114d5fc45f5299c3b2011270034a3370e0ec388 we
were no longer registering defaults for cinder.api.common.py settings.

The result is that you're unable to run individual unit tests like
cinder.tests.api.test_volume_metadata.py.

Register the needed CONF options and we're all set again.

Fixes bug: 1194315

Change-Id: I6df6c5ec5687c8156a9f3794cc7f04d8d28065d9
Merge "Register used CONF entries in cinder.api.common.py"