Actually create /etc/neutron/plugin.ini

Somehow this symlink was lost, so neutron server failed to start without
any useful diagnostics (it went to stderr which was redirected to
/dev/null).

To address the problem we:
- cleaned up and fixed code that returns config file name in plugin
  configurators;
- renamed properties that returns the path so that they do not start
  from 'get_';
- implemented the property in both supported core plugins (linuxbridge
  and openvswitch) and removed it from non-core plugins;
- added code to actually create the symlink at post_install phase.

Change-Id: Ia0ce1a3dd156914870a975a893f32e77c9330476