BVS-828 Add OVS installation to install-node-havana script

This purges all openvswitch packages and then adds the following
(along with their dependencies):

* openvswitch-datapath-lts-raring-dkms   1.9.0-0ubuntu1~ubuntu12.04.1                Open vSwitch datapath module source - DKMS version
* openvswitch-datapath-lts-raring-source 1.9.0-0ubuntu1~ubuntu12.04.1                Open vSwitch datapath module source - module-assistant version
* openvswitch-switch                     1.4.0-1ubuntu1.5                            Open vSwitch switch implementations