A gzip file doesn't have all the "archive" properties that a tarfile
or zipfile has, but still, it could be useful to wrap in the archive
module.

At this point, support is being added to allow for files compressed
with gzip, to be acknowledged as valid in avocado.utils.is_archive()
and transparently handled with avocado.util.uncompress().

The goal of this implementation is to allow for gzip files, which
are compressed tarbals, to be properly handled at uncompress() time.
Signed-off-by: NCleber Rosa <crosa@redhat.com>