Adds to KMIP secret store test coverage

Adds a few small changes to make the KMIP Secret Store plugin more
robust, along with unit tests that raise the kmip_secret_store test
coverage to 100% (once pykmip is added test-requirements.txt)

Change-Id: Id3772e05e269a8d96c7205ade49d429e5fc0b50f
Merge "Adds to KMIP secret store test coverage"