getKeyVault.createKey() should return the created data key

In 4.2.0, the getKeyVault.createKey() method returns a WriteResult instead of the created key. This requires users to do one of two things:

• Search by key alternate name for the created key, or
• Search all keys in reverse creation order for the most recently created key

As noted in the spec document, data key creation methods should return at least the _id of the created document.

The 4.2 shell should either return the _id of the created data key per-spec.

Relevant code here

cc shreyas.kalyan