Polish 'Savanna Deployment' page

That incorporates feedback given by Meg McRoberts in
https://review.openstack.org/#/c/71299/4/pages/reference-architecture/0090-savanna.rst

I didn't have time to make this change back then, but I think it would be
nice to have it in 5.0.

Change-Id: I027eab13dc5367fbc74b14444a4da1fdb11f52e4