vpp-agent doesn't start with Ocata

[Martin Ivanov]

When starting vpp-agent with neutron ocata following components:
neutron==10.0.4
neutron-lib==1.1.0
Following error is seen:
Traceback (most recent call last):
  File "/usr/bin/vpp-agent", line 6, in <module>
    from networking_vpp.agent.server import main
  File "/usr/lib/python2.7/site-packages/networking_vpp/agent/server.py", line 46, in <module>
    from networking_vpp import compat
  File "/usr/lib/python2.7/site-packages/networking_vpp/compat.py", line 66, in <module>
    import neutron.plugin.common.constants
ImportError: No module named plugin.common.constants

[Ian Wells (ijw-ubuntu)]

Compat fix, long long ago...