Specifics for nova install in Debian.

This patch removes things which are not needed to be in
the Debian installation chapter for Nova, and adds some
Debian specifics (plus remove the "Notes for Debian
users).

Note that this is a break-down of:
https://review.openstack.org/54394/

into smaller patches.

backport: havana

Change-Id: I3c0f538acb392194733293478fa35e9824328fff
Merge "Specifics for nova install in Debian."