Acceptance Tests: rename attribute holding the distro image checksum

Acceptance Tests: rename attribute holding the distro image checksum

This renames the attribute that holds the checksum for the image Linux
distribution image used.

The current name of the attribute is not very descriptive.  Also, in
preparation for making the distribution used configurable, which will
add distro related parameters, attributes and tags, let's make the
naming of those more uniform.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Message-Id: <20210414221457.1653745-2-crosa@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[CR: split long lines]
Signed-off-by: Cleber Rosa <crosa@redhat.com>