Revise admin interoperable image import docs
Revise admin interoperable image import docs

This patch revises the documentation of the interoperable image import
feature available to admin operators to concisely describe Stein and
later releases.

Changes include:
- Remove enable_image_import option because it is no longer available
  since the release of Stein
- Simplify the language of the v1 API being deprecated

Change-Id: Ic155afd6de5b37a25743457e9a7ddd5a45dac4e8
Closes-Bug: #1808814

openstack/glance,openstack/glance,openstack/glance