objectstore: trivial cleanup in _Base
objectstore: trivial cleanup in _Base

The _iter_accounts() method should use self.CACHE_KEY_HEAD instead of the
currently hardcoded "swift.head_account".

Change-Id: Ie95e1f2875be3680bb76c209dabd8c91f5d78234

luogangyi/Ceilometer-oVirt,r-mibu/ceilometer,pczerkas/aodh,cernops/ceilometer,openstack/ceilometer,mathslinux/ceilometer,idegtiarov/ceilometer,tanglei528/ceilometer,tanglei528/ceilometer,ityaptin/ceilometer,citrix-openstack-build/ceilometer,sileht/aodh,eayunstack/ceilometer,Juniper/ceilometer,fabian4/ceilometer,rackerlabs/instrumented-ceilometer,isyippee/ceilometer,NeCTAR-RC/ceilometer,MisterPup/Ceilometer-Juno-Extension,chungg/aodh,luogangyi/Ceilometer-oVirt,rackerlabs/instrumented-ceilometer,redhat-openstack/ceilometer,maestro-hybrid-cloud/ceilometer,mathslinux/ceilometer,eayunstack/ceilometer,froyobin/ceilometer,m1093782566/openstack_org_ceilometer,m1093782566/openstack_org_ceilometer,cernops/ceilometer,openstack/aodh,Juniper/ceilometer,citrix-openstack-build/ceilometer,rackerlabs/instrumented-ceilometer,pczerkas/aodh,tanglei528/ceilometer,openstack/aodh,JioCloud/ceilometer,citrix-openstack-build/ceilometer,froyobin/ceilometer,JioCloud/ceilometer,pkilambi/ceilometer,redhat-openstack/ceilometer,idegtiarov/ceilometer,rickerc/ceilometer_audit,isyippee/ceilometer,rickerc/ceilometer_audit,MisterPup/Ceilometer-Juno-Extension,ityaptin/ceilometer,sileht/aodh,r-mibu/ceilometer,rickerc/ceilometer_audit,NeCTAR-RC/ceilometer,JioCloud/ceilometer,fabian4/ceilometer,openstack/ceilometer,pkilambi/ceilometer,chungg/aodh,NeCTAR-RC/ceilometer,maestro-hybrid-cloud/ceilometer