[install-guide] Update prerequisite keystone procedure
[install-guide] Update prerequisite keystone procedure

Remove mention of the admin token and update numbered list
formatting.

Change-Id: I4d5ddc177b44a555f26fe14b75cfe654b292ba89
Closes-Bug: #1663785
backport: newton

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