Standard attributes (i.e. revision_number, created_at, updated_at)
are missing in the segment resource. This is because the controller
doesn't process extensions on rendering the response. This patch
fixes it. In particular, this patch does the following:
* Process extensions when making the segment dict. This allows
other extensions to add attributes to segment response.
* Make segment extension implement update_attributes_map.
Tempest test: https://review.openstack.org/#/c/558609/
APIImpact update api reference of segments
Closes-Bug: #1760902
Change-Id: I3c93818002c2d7753454547231ba08544b6fa1c0