Include okd-rpms in the payload, teach Assisted Installer to use it automatically

In order to install OKD via Assisted Installer currently an additional configuration option - `OKD_RPMS` is required. This image was previously built manually and uploaded to quay.

It would be useful to include it in the payload and teach Assisted Service to extract it automatically, so that this configuration change would not be required. As a result, the same Assisted Installer can be used to install both OCP and OKD versions. Implementing this would also simplify agent-based cluster 0 installation.

cc andrea.fasano