The openSUSE OBS OpenStack repositories provide rabbitmq-server and
erlang packages tested in an openSUSE OpenStack cloud environment so
it's best to use these instead of upstream ones. As such we change the
default option to be 'distro' instead of 'file'. However, for the tests
we use 'file' so we can install an older version and test the upgrade
path. Moreover, we drop the 'zypper lock' code since the OBS repositories
only provide one version of the needed packages. Finally, we fail if the
installation method is set to external since there is no upstream
repositoiry for openSUSE.
Change-Id: I79b8f0af97b6fcbf904f4b497a8ab0e070d20542