Deprecate and remove libvirt.api_thread_pool option

[Matt Riedemann]

We should deprecate the libvirt.api_thread_pool config option in icehouse and remove it in the J release. Looks like the last time it was touched was in January for some refactoring:

https://github.com/openstack/nova/commit/ce27bca5497600ff9ab7195e27ede94e7cffe5d0

The reasons why this should be removed are detailed here:

https://review.openstack.org/#/c/57000

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/59941
Committed: http://github.com/openstack/nova/commit/22cd7cc27ca8285c1674e370e256ec84d3f2a20a
Submitter: Jenkins
Branch: master

commit 22cd7cc27ca8285c1674e370e256ec84d3f2a20a
Author: Michael Kerrin <email address hidden>
Date: Mon Nov 25 11:03:26 2013 +0000

    Remove the api_thread_pool option from libvirt driver

    This gave us the option to select if calls to libvirtd are done
    in a native thread or directly. This was added because the person
    who wrote the thread pool code was afraid of regressions for existing
    deployments. But now we have enough features in nova that require
    that this is always set to True or else users can have issues with
    them. To this point the firewall overrode this option to make
    sure that its call was always in a native thread no matter the
    value of this option was.

    The reasons behind this were discussed during this review:
    https://review.openstack.org/#/c/57000/

    Closes-Bug: #1254811

    DocImpact: removes api_thread_pool flag from the libvirt group

    Change-Id: Ifad7f1bdf2af69e4fcd6f9a04bf2f7ff5251afd6