Change ``ipa_build_dib_elements_default`` and
``ipa_build_dib_env_default`` to use ``os_distribution`` and
``os_release`` by default. This allows for Ubuntu images to be built
when running on Ubuntu.

Rocky will still build CentOS images, as Rocky IPA images have not been
tested yet.

Change-Id: Iefd2d0b7a3a3e07f5c112d58e2ec0b3da0a747d3