This runs selenium from a container on a node, and exposes port 4444
so you can issue commands to it. This is used in the follow-on
I56cda99790d3c172e10b664e57abeca10efc5566 to take some screenshots of
gerrit.
Change-Id: Idcbcd9a8f33bd86b5f3e546dd563792212e0751b
