Find major version from stream, avoid mismatches

With the current build-snap jobs, it's possible to pass a major version
that doesn't match the stream. This results in building an RPM from the
given major version but pushing it to the Nexus repo for the stream.

Don't accept version params and always derive them from job stream.

Change-Id: I471c7e5e8aca09a45859d88abb4200b6a825ca64
Signed-off-by: Daniel Farrell <dfarrell@redhat.com>