Add missing conversion specifier to ServiceGroupUnavailable
Add missing conversion specifier to ServiceGroupUnavailable

The message in the ServiceGroupUnavailable exception was missing the 's'
conversion specifier.

Fix bug 1183733

Change-Id: I54869431511a72c49ccf61f5e7b1a30607959423

openstack/oslo.versionedobjects,citrix-openstack-build/oslo.versionedobjects