provision/docker: create addNodeToSchedulerCmd

This will be a command used from the command line interface to
register new nodes within the scheduler.
provision/docker: simplify test
provision/docker: register addNodeToSchedulerCmd
provision/docker: create removeNodeFromSchedulerCmd

This command will remove nodes from the scheduler.
provision/docker: unexport scheduler errors
provision/docker: register removeNodeFromSchedulerCmd in the provisioner