Cleanup unused settings, use interface_driver setter

* Remove unused MIDONET specific settings from nova.conf
* Remove unused MIDONET specific settings from dhcp_agent.ini
* Move the interface_driver settings to the proper setting
  function so that it can be used by lbaas_agent.ini as well.

Change-Id: Id686ff5f55db00bce42b8a2bd56e7655c73211fb
Closes-Bug: #1245401
(cherry picked from commit e4a523f543e59d92ab227b5bcfaad09fc171b6a9)