Note that XML support *may* be removed.

To be more accurate, note that XML support *may* be removed as early
as the Juno release.  I think there is still more discussion needed
around concrete usage data before the removal date is finalized.

Related thread:
http://lists.openstack.org/pipermail/openstack-dev/2014-April/031608.html

Closes-bug: #1301384
Change-Id: I0415b50ec0b81bb56f5c0fa13bc6d01f8bec7865
(cherry picked from commit 5d39189df6ecb559c3f8b7e2fa3beff25da9f452)
Merge "Note that XML support *may* be removed." into milestone-proposed