json.at: Avoid to run $PYTHON when python is not available

json.at: Avoid to run $PYTHON when python is not available

This commit is needed since autotest tries to run AT_XFAIL_IF when
AT_SKIP_IF condition is true too.

Currently HAVE_PYTHON is required, but this may change in the future
since many distributions are migrating to python3 as default python
(like Arch Linux or Fedora) and so it can be a good idea to permit to
build OVS using python3.

Signed-off-by: Timothy Redaelli <tredaelli@redhat.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>