qa/tasks/openssl_keys.py: sort cert configs before creating certs

qa/tasks/openssl_keys.py: sort cert configs before creating certs

we cannot rely on the order in which items are arranged in a dict, the
order varies from version to another. in Python2, it happens to work,
and we can always have the self-signed cert added first. but in Python3,
it does not. and an exception is thrown
```
teuthology.exceptions.ConfigError: ssl: ca root not found for
certificate rgw.client.0
```

in this change, before creating certs, the settings are reordered so
that the self-signed ones are created first.

Signed-off-by: Kefu Chai <kchai@redhat.com>