After three months since the quality warning change merged [1]
there has still been no progress in finding a maintainer for
the xenapi driver or someone to get the third party CI running
again - which has been off/broken for more than a release.
This change formally deprecates the driver, logging a warning
on startup along with providing a release note and warnings
in the docs and [xenserver] config group help.
Note that this does not mean the driver will absolutely be
removed in the Ussuri release, but it leaves the option open
to do so if the nova team decides that should happen.
This was discussed at the 2019-09-05 nova meeting [2] and
also at the Train PTG.
[1] I7f8eb7d5c5a9b1cb0a8d5e607d719b49a22675d3
[2] http://eavesdrop.openstack.org/meetings/nova/2019/nova.2019-09-05-14.01.log.html#l-227
Change-Id: Ie7e66ff64185d9fd4be2265e040e1afc45b95174
af280ffe30