Fixes to some corner cases + support for some missing 'options and

Fixes to some corner cases + support for some missing 'options and
attributes' for backward compatibility

Ticket: CM-1438
Reviewed By:
Testing Done: Tested ifupdown sanity and new functionality

support for:
- -i <interface file>
- template lookup path and move all template handling to a separate
  module template.py
- new ifupdown2 config file /etc/network/ifupdown2/ifupdown2.conf
- bridge_waitport and bridge_maxwait
- moved addons.conf to /var/lib/ifupdownaddons/