Make it possible to create a user via API

When POSTing to the /users URL, wrap data within an extra hash. This is per the
API document.

    $ python -m unittest tests.foreman.api.test_user
    ....s......s......s.....s......s......s.
    ----------------------------------------------------------------------
    Ran 40 tests in 79.635s

    OK (skipped=6)

Targets #1578.
Merge pull request #1585 from Ichimonji10/create-user

Make it possible to create a user via API