Debian specifics for keystone

This patch adds Debian specifics for the Keystone install
chapter.

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

into smaller patches.

backport: havana

Change-Id: I9c7383f3daffab06fbe12d35208923eda00b5207
(cherry picked from commit 3c42bf4f1ba08adb0496476af52f03ec4e6ec858)
Merge "Debian specifics for keystone" into stable/havana