Enable multinode primitive for docker test shell.
We need to use
lava-wait for a job which has docker-test-shell action, meanwhile it's a multinode job.
DockerTest has priority = 10, while
MultinodeTestShell has priority = 2, which means
role be ignored here, then when call
lava-send etc. They won't connect to lava-coordinator to exchange message.
This patch separate multinode behavior to mixin to let other actions utilize the mixin.