autotest_firewalld_add_service_unittest: fix #! to use /usr/bin/env

/usr/bin/env python is preferrable because it supports virtualenv.

Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>
autotest_firewalld_add_service_unittest: resolve pylint by splitting unit test

Break test_get_default_zone into two separate unit tests and thus avoid pylint
complaints about '_' being defined twice.

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