Use of logging in native thread causes deadlock connecting to libvirtd

When connecting to libvirtd daemon we were calling the _connect
method in a native thread. The use of logging in this method hit this
bug in eventlet https://bitbucket.org/eventlet/eventlet/issue/137/
causing nova-compute to hang.

Closes-Bug: #1252409

Conflicts:
        nova/virt/libvirt/driver.py

NOTE(mriedem): The conflict is due to set_host_enabled
being used in the original change in Icehouse and the
option rename for libvirt_nonblocking.

Change-Id: I0d22c3ed1b3288aeb1afae130f11a4f38dc6440a
(cherry picked from commit e4b0d8e9446ad3ad3a514b9cf65722f6ed95888d)
Merge "Use of logging in native thread causes deadlock connecting to libvirtd" into stable/havana