Beaker shouldn't enforce any authconfig.

Use the default authconfig unless otherwise specified
by the user using the ks_meta option.

As a side effect, for RHEL 6, the job submitter can specify
an authconfig and it will override the default. Earlier, this
wasn't possible.

Bug: 989924
Change-Id: I713dc40ccd0f654c22ac96bce1c9d2cf5bf85b86