Typically, the Ironic API client in IPA will autodetect the API version
based on the output of a GET of the root of the API. If for some reason
this API endpoint is restricted, or the operator wishes to limit the
Ironic API version IPA uses, they can now set CONF.ironic_api_version to
avoid autodetection and force a version.
Change-Id: Ib96a1057792f45f2e4554671e32c436140463ee8