Update test to reflect changes

When an HTTP request is sent to a Foreman server with a content-type of
text/plain, an HTTP 415 (unsupported media type) response is received. This is
correct behaviour: only text/json messages are supported by Foreman. Update the
test suite to reflect this change.
Merge pull request #1143 from Ichimonji10/jenkins

Update test to reflect changes
Allow Factory to accept explicit auth credentials

Update the following methods:

* robottelo.factory.Factory.build
* robottelo.factory.Factory.create

Add optional argument `auth` to both methods. Clients may provide a
`('username', 'password')` pair to this argument. If clients do this, then those
credentials will be used when interacting with the API instead of whatever
credentials are returned by function `get_server_credentials`.

Also: update some docstrings to be nicer, clean up some tests using the mock
library, and add a new test for `Factory.create`.
Improve API smoke test

The ability to override default authentication credentials makes some code
unnecessary. Remove it.

Slightly expand the API smoke test. Doing this helps to demonstrate that the new
method of specifying credentials is working, and it is a step down the road of
implementing an API smoke test.