autotest-firewalld-add-server: convert to Popen and add unit test

The docs say: subprocess.Popen is preferable to commands.  Also convert
_get_default_zone to static method so we can unit test it without
argparse.

v2: use standard autotest common import

Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>
autotest-firewalld-add-service: improve handling of missing default zone

The print_help() message wasn't useful when this script is run without
firewall-cmd present.
Change the _get_default_zone to return None is firewall-cmd fails, then
add a specific error message indicating that we can't find the default
zone.

Also remove the 'not (zone and service)' check, since we check each
individually.

Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>
autotest-firewalld-add-service: exit with status 1 when missing args

Don't exit with success help missing an argument.

Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>
autotest-firewalld-add-service: convert methods to static

The new PyCharm 3.0 will now suggest conversion to staticmethod when
a method doesn't use self, so I decided to try it out.

Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>