Refactor check_tags to account for optional tags
Refactor check_tags to account for optional tags

check_tags previously checked that no tags besides those specified were present.
It now accounts for optional tags; tags which are not a cause for failure if
present or absent, as long as they are not empty.

The updated interface is backwards-compatible.

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