Add cfg.CONF.import_group for service_credentials

The cfg.CONF.service_credentials attribute is used without calling
cfg.CONF.import_group. Add calling the method in this patch.

Change-Id: I700c5ea7ae5ff3f61c28a767e5aa60db0dfb46ce
Closes-Bug: 1373753
Merge "Add cfg.CONF.import_group for service_credentials"