`[keystone]` should be `[keystone-base]` in doc
`[keystone]` should be `[keystone-base]` in doc

This can be misleading when building source-type images, as `[keystone]`
doesn't work as expected.

Change-Id: Id59fa5f05e4ddeb4ac0afc111a4fc516c503e58b

intel-onp/kolla,nihilifer/kolla,intel-onp/kolla,stackforge/kolla,nihilifer/kolla,openstack/kolla,openstack/kolla,rahulunair/kolla,GalenMa/kolla,mrangana/kolla,stackforge/kolla,stackforge/kolla,rahulunair/kolla,GalenMa/kolla,mrangana/kolla