Install Guide: Fix Open vSwitch plugin/Ubuntu

It should be neutron-plugin-openvswitch-agent on Ubuntu/Debian

Change-Id: Ib3e081c657657d3395beeda01681a532ac66ee00
backport: havana
Closes-Bug: 1246042
Partial-Bug: 1245914
Merge "Install Guide: Fix Open vSwitch plugin/Ubuntu"