Start linuxbridge neutron agent using a launcher

Start linuxbridge neutron agent using a launcher

This change ports linuxbridge neutron agent on common/service code
that will allow to handle termination signals (SIGHUP, SIGTERM, SIGINT).

Note that this patch allows changing only logging options.
All other config options are not handled explicitly and changing
them using SIGHUP can lead to unpredictable circumstances.
So, until changing other options is handled it is highly recommended
to use SIGHUP for changing ONLY logging options.

DocImpact
Change-Id: I1d834e0683b04111ef04c148cbd8d4acf2964065
Closes-Bug: #1461539