Add support for managing async processes

Interacting with a long-running asynchronous process requires the
use of non-blocking io.  This change adds a helper class that can
launch a long-running process and read stdout and stderr in a
non-blocking fashion via eventlet.

This functionality is intended to support monitoring ovsdb via
a long-running and root-privileged invocation of ovsdb-client.

The complexity of the system interaction in this patch suggested
the addition of a functional test that validated actual behaviour.
The test was added under the neutron/tests/functional path which
is now included in the testr search path.

Partial-Bug: #1177973

Change-Id: I9969e556acecf7a9e77d873371cc2ec2647be011
(cherry picked from commit acf0209b28e21eed60158967fab77468eb195e7c)
Merge "Add support for managing async processes" into stable/havana