Clarify test case in common/ring/test_builder
Clarify test case in common/ring/test_builder

They use a bare assertRaises(ValueError, ring.RingBuilder, *,*,*), but
it's not clear which one raises which ValueError(), so I extend them to
validate the error strings as well.

Change-Id: I63280a9fc47ff678fe143e635046a0b402fd4506

openstack/swift,psachin/swift,clayg/swift,prashanthpai/swift,swiftstack/swift,nadeemsyed/swift,matthewoliver/swift,clayg/swift,nadeemsyed/swift,matthewoliver/swift,matthewoliver/swift,psachin/swift,tipabu/swift,matthewoliver/swift,prashanthpai/swift,hurricanerix/swift,tipabu/swift,smerritt/swift,psachin/swift,notmyname/swift,openstack/swift,nadeemsyed/swift,clayg/swift,hurricanerix/swift,clayg/swift,tipabu/swift,redbo/swift,nadeemsyed/swift,hurricanerix/swift,notmyname/swift,openstack/swift,notmyname/swift,swiftstack/swift,smerritt/swift,psachin/swift,smerritt/swift,notmyname/swift,smerritt/swift,hurricanerix/swift,tipabu/swift,swiftstack/swift,redbo/swift,openstack/swift