Acceptance Tests: Add default kernel params and pxeboot url to the KNOWN_DISTROS...

Acceptance Tests: Add default kernel params and pxeboot url to the KNOWN_DISTROS collection

When running LinuxTests we may need to run the guest with
custom params. It is practical to store the pxeboot URL
and the default kernel params so that the
tests just need to fetch those and augment the kernel params.

Signed-off-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20210706131729.30749-3-eric.auger@redhat.com>
[CR: split long lines]
Signed-off-by: Cleber Rosa <crosa@redhat.com>