No need to edit glance paste files

As of Folsom, there is no need to edit the glance-api-paste.ini and
glance-registry-paste.ini files in a regular install of glance. Remove
references to this in the install doc.

For implementation details, see:
 * https://bugs.launchpad.net/glance/+bug/1043490
 * https://review.openstack.org/12149
 * https://github.com/openstack/glance/commit/b7197041f

Change-Id: Ic3ef513f818727a7daebda2b7271e5dcd66ffe91