This sets up a default 'br-infra' bridge between nodes in a multi-node
job. Two groups are expected in order for this to be set up:
- switch
- peers
The 'switch' group, with only one expected member, will be set up as
the switch node while the 'peers' group will be connected to the switch
node with virtual ports.
The default settings such as the bridge name, the IP range used, the MTU
and so on can be overridden at the job level by setting vars.
Change-Id: If5a3f512837e5b235bd2f6dbfdbb454435aa983e
645a3935ee