fix the issues with the recent refactoring and change in imports

In this changeset following issues have been fixed:

1. _LE and _LI are now imported from neutron.i18n
2. _LE and _LI accepts only a single argument, unlike a tuple
   previously
3. Fixes failing test scripts with a special patch for @property
   of core_plugin in router service plugin
4. This also fixes delete router failing tests because of the
   patching done using an incorrect class