ssl_ca in ssl_utils.py double encoded when using external ca

[Martin Hellström]

When using the ssl_ca option on the rabbitmq the ssl_ca relation data will be base64 encoded twice. This is because the charm option should be a base64 encoded string according to the config.yaml

  ssl_ca:
    type: string
    description: |
      Certificate authority cert that the cert. Optional if the ssl_cert is
      signed by a ca recognized by the os. Format is base64 PEM (concatenated
      certs if needed).

And later on when the relation_data is set in 'hooks/ssl_utils.py' the string is base64 encoded again:

        if config('ssl_ca'):
            relation_data['ssl_ca'] = base64.b64encode(
                config('ssl_ca'))

This prevent clients from connecting to the rabbitmq cluster since they only decode it once.