Correctly set the L3 service plugin for ML2

ML2 uses a service plugin for L3. This patch to devstack
correctly sets this by setting or updating the variable
Q_SERVICE_PLUGIN_CLASSES, which makes ML2 compatible when
running with other service plugins (e.g. LBaaS and VPN).

Fixes bug 1231622

Change-Id: I0ce1f5a42bd052995135ffac1ee5ef382d69789e
Merge "Correctly set the L3 service plugin for ML2"