PLUMgrid plugin decomposition part II

PLUMgrid plugin decomposition part II

As part of the phase II plugin decomposition [1],
this change moves PLUMgrid plugin to its new
home: networking-plumgrid [2]

[1] http://docs.openstack.org/developer/neutron/devref/contribute.html
[2] https://github.com/openstack/networking-plumgrid

Partial-Implements: blueprint core-vendor-decomposition
Change-Id: Ifc6bbb4fe45ee9b8d298ff171ee43c0da37d075c
Signed-off-by: Fawad Khaliq <fawad@plumgrid.com>