Add router_id in dr_agent.ini

This patch modifies the way to specify the BGP identifier of the BGP
speaker puting it in the configuration file, dr_agent.ini.

Change-Id: Iafd4edde40c5c34dd2c1ebad28ca6417d3001de1
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>
Separate LinuxBGPDriver and RyuBGPDriver

This patch renames neutron/agent/linux/bgp.py to
neutron/agent/linux/base.py and separates LinuxBGPDriver from
RyuBGPDriver because RyuBGPDriver requires Ryu to be imported
in the header only just once. Otherwise it causes
ArgsAlreadyParsedError.

Change-Id: Ib282e02a777bd412ec281253f326dea0ca854f6d
Signed-off-by: Taku Fukushima <tfukushima@midokura.com>