autotest regression test: default to master branch

Since the next branch does not exist anymore.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
autotest regression test: fetch the installer script from the chosen branch

If using the github repo URL. If not, default to the script on the github
master branch.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Merge pull request #1195 from clebergnu/autotest_regression_branch

Autotest regression test