Multiple L3 Agents

Added "L3 Agents" article to this commit to include some
background information about this feature.

Added note to ops-guide/troubleshoot/9100-tshoot-corosync-pacemaker
about the clone_p_neutron-l3-agent resource

Added "New Feature" article to the Release Notes

Change-Id: Id20a2a5a03a5f67e7e7619cf71fa9b088ed50490
Merge "Multiple L3 Agents"