Update nova docs front page for placement removal
Update nova docs front page for placement removal

Remove links to internal placement documentation. Instead indicate
that placement is required by nova, and point elsewhere.

The depends-on adds placement install docs to the placement repo.

Depends-On: https://review.openstack.org/628220
Change-Id: I9f0d52e7b46b270363946211cf6dc0c338981cb2

klmitch/nova,klmitch/nova,klmitch/nova,mahak/nova,rahulunair/nova,mahak/nova,openstack/nova,rahulunair/nova,mahak/nova,openstack/nova,klmitch/nova,openstack/nova,rahulunair/nova