Fix a typo in log exception in the metering agent

This patch fixes a typo when there is a exception
when the agent try to invoke a method of the driver.
This patch also improves exceptions handling of
driver method calls.

Change-Id: I4fcf19fa84f178348abfb3563bbfd52f0dfcc095
Closes-bug: #1256041
(cherry picked from commit 272912b6d82af00a6f7bafc26d8ce7a79b09ef26)
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)
Fix metering iptables driver doesn't read root_helper param

With this patch the iptables driver instantiates the
iptables_manager with the correct root_helper value.

Change-Id: I9a37bae92dc12a8a78ea2d1b9fc5e995321ca322
Closes-bug: #1256036
(cherry picked from commit 35f64f8026f7607c749727ac6b3ee93c956bdfd2)
Merge "Fix metering iptables driver doesn't read root_helper param" into stable/havana
Merge "Add support for managing async processes" into stable/havana
Merge "Fix a typo in log exception in the metering agent" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable