Add an environment variable to enable extensions in keystone

Following https://review.openstack.org/#/c/44401/3

Change-Id: I8e0391cd68e29e6d57c0f35517bf4155110861c7
Closes-Bug: #1218733
Co-Authored-By: Yong Sheng Gong <gongysh@unitedstack.com>
Merge "Add an environment variable to enable extensions in keystone"