Install Guide: Fix nova.conf for nova-network

These fixes are needed for Ubuntu and openSUSE/SLES:
Setup nova.conf properly
Start libvirtd (openSUSE/SLES)

Change-Id: Ied6580fbbde85bf86e8d1986dc5e94e7a54b27d7
backport: havana
Closes-Bug: 1245846
Closes-Bug: 1245869
Merge "Install Guide: Fix nova.conf for nova-network"