This patch adds support for reading the 'enable-qos' setting from the
neutron-plugin-api relation and adding 'qos' to the extension_drivers setting
if it is True. This is part of a wider set of changes to support QoS across the
neutron charms.
A charmhelper sync was performed to pull in the QoS update to the
NeutronAPIContext.
Note: Amulet tests will fail until the corresponding neutron-api change
lands
Depends-On: I1beba9bebdb7766fd95d47bf13b6f4ad86e762b5
Change-Id: I9d857a4f2a25c6080963a0f3f6e6592c0a77d133
Partial-Bug: #1705358
cf383ecd3f