This adds the required framework to test the 'multinode' job and adds
a first integration tested role: multi-node-known-hosts.
The groups 'switch' and 'peers' are bound to be used when integration
testing the 'multi-node-bridge' role.
Change-Id: Ia84189ce3e1b273ca8509d2a9796970750191977
Depends-On: Ia7480dab0f357b5710d1962a40499381837942b3
c588aebca3