Some Neutron plugins require controllers and multiple backend services
to operate correctly. This patch adds the framework for third party
plugins to run sanity checks after Neutron Server has started.
This simple addition may reveal potential configuration pitfalls
much earlier in the dev/test cycle, thus speeding up the build
churn process.
The first plugin that uses this framework is the VMware NSX one.
Closes-bug: #1265671
Change-Id: I17f9c5c8e828316ff03f0eff42ae4ae6c6c58733
ef1e08022b