Add support for setting extra networking configuration options.

Add support for generically setting arguments for OpenStack Networking
for both the server and agent. This is done using Q_SRV_EXTRA_OPTS for the
server, which let you place settings in the "OVS" or "LINUX_BRIDGE"
sections of the plugin configuration file. For the agent, two variables
are defined: Q_AGENT_EXTRA_AGENT_OPTS lets you set items in the "AGENT"
section of the plugin configuration file, and Q_AGENT_EXTRA_SRV_OPTS
lets you set things in the "OVS" or "LINUX_BRIDGE" sections of the
plugin configuration file.

Implements blueprint devstack-quantum-config.

Change-Id: I9ba44b21231f88cda1bcc5e3389c7875f03d8145
Enables the multibackend Cinder tests in tempest when useful.

The change adds in lib/tempest an optional dependency on the var
$CINDER_MULTI_LVM_BACKEND which, if set, enables the multibackend
tests in the tempest config.

Change-Id: Iccafd00f5adabbbc2309fa72664bf29440744d91
Merge "Add support for setting extra networking configuration options."