Add response schema validation for volume messages

This is to add response schema validation for volume
messages, also to remove the fields check in testcases.

NOTE: For the message_level field, possible value is only
‘ERROR’ now, but may be extended later, so we do not restrictly
check the value here.

Change-Id: I1c404c942ef9c5c96aa898721487d891f8795eb4
partially-implements: blueprint volume-response-schema-validation
3 files changed