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
Merge "Remove the api_thread_pool option from libvirt driver"