Add a note that Xen section is out of date

Patch adds a note that the Xen section is out of date, and
links to the blueprint to fix it. Xen still looks to devstack
for install items, and hasn't been updated for three releases
or so.

Change-Id: Ib2fa68e307cf465f496c43d345a5aa3a052d43fe
bp:redocument-xen
tox.ini: Whitelist rm

Remove noise from execution, whitelist rm command.

Change-Id: I270d7e98fe857501fa6ac1a38f6ff928ea0192de
Merge "tox.ini: Whitelist rm"
Merge "Add a note that Xen section is out of date"