Image client v1 and v2 try to auth in __init__

The image clients try to auth against the identity service at __init__ time.

This is inconsistent with what all other clients do.
Even if not running glance tests, even if glance is set to false in the service_available config group, the glance client will try to authenticate.

    def __init__(self, config, username, password, auth_url, tenant_name=None):
        super(ImageClientJSON, self).__init__(config, username, password,
                                              auth_url, tenant_name)
        self.service = self.config.images.catalog_type
        self.http = self._get_http()

This can be solved for instance by definiting get_http and calling that in place of self.http, or by using __getattr__ to handle the case of self.http not being defined yet.

e.g.

  def get_http(self):
      return self.http if self.http is not None else self._get_http()

or

    def __getattr__(self, name):
        if name == 'http':
            self.http = self._get_http()
            return self.http
        else:
            raise AttributeError(name)