Fix tests for schema 0.7.0
Fix tests for schema 0.7.0

The 0.7.0 release of the schema package changed the wording in some
error messages. This updates tests to reflect the change.

kensipe/dcos,GoelDeepak/dcos,dcos/dcos,mesosphere-mergebot/dcos,kensipe/dcos,mesosphere-mergebot/mergebot-test-dcos,kensipe/dcos,GoelDeepak/dcos,mesosphere-mergebot/dcos,dcos/dcos,mesosphere-mergebot/mergebot-test-dcos,dcos/dcos,mesosphere-mergebot/mergebot-test-dcos,GoelDeepak/dcos,mesosphere-mergebot/mergebot-test-dcos,mesosphere-mergebot/dcos,dcos/dcos,mesosphere-mergebot/dcos,kensipe/dcos,dcos/dcos,GoelDeepak/dcos