vpp l3 plugin should avoid dependency on vpp_papi

[Hareesh Puthalath]

Vpp l3 plugin has a dependency on vpp agent for reading the etcd namespace keys [LEADIN, ROUTERS_DIR etc]. For this it imports vpp agent modules (server.py, vpp.py) which introduces a dependency on vpp and vpp_papi.py

python -c "from networking_vpp.agent import server"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/usr/lib/python2.7/site-packages/networking_vpp/agent/server.py", line 44, in <module>
    import vpp
  File "/usr/lib/python2.7/site-packages/networking_vpp/agent/vpp.py", line 28, in <module>
    import vpp_papi
ImportError: No module named vpp_papi

In distributed scenarios where neutron server runs in its own container/controller independent of the agent, this dependency is avoidable.

A refactoring is needed to move this attributes to a common class.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/545985
Committed: https://git.openstack.org/cgit/openstack/networking-vpp/commit/?id=d8c2f8d862b9928519edd26b5061cc43499f9298
Submitter: Zuul
Branch: master

commit d8c2f8d862b9928519edd26b5061cc43499f9298
Author: Naveen Joy <email address hidden>
Date: Mon Feb 19 13:02:54 2018 -0800

    Create a common constants module to avoid unwanted dependencies

    Move all the shared constants between the server, agent and the
    plugins to a common constants file to avoid unnecessary dependencies
    between modules.

    Change-Id: I89573b22e0ff246ff4fe607af207a9226045e7bb
    Closes-Bug: #1750255