Creating symbolic link for plugin.ini

We need create the link for plugin.ini on rhel, centos, fedora
platforms, otherwise the neutron-server can not be started. Here is the
bug from community:
https://bugs.launchpad.net/openstack-manuals/+bug/1240697

Closes-Bug: #1268670
Change-Id: I6f0fdc0a7610ba55e8cd7051805fd5883e41c87e