[install] Update Horizon dependency
[install] Update Horizon dependency

In Newton release, horizon's dependency is now only keystone.
It means, horizon supports swift only deployments with keystone.

https://review.openstack.org/#/c/371161/ update the install guide
but this file was not covered.

Horizon release note:
http://docs.openstack.org/releasenotes/horizon/unreleased.html#b3

Change-Id: I09d2ebaebc0473f03e97fc671a2ff07eb99877fc

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals