Update property keys document
Update property keys document

The list of useful image properties really belongs in the Glance
Administration Guide.  The only connection they have with the
glanceclient is that you can use it to set them.  Rewrite the
property keys document to reflect this.  The dependency moves the
relevant text to the Glance Admin Guide.

Depends-on: https://review.openstack.org/565780
Change-Id: I39add6728aad42988a07d2ad97cd81dbdaf4c54a

openstack/python-glanceclient,openstack/python-glanceclient