Port client portion of #4392 to new taskrunner
Port client portion of #4392 to new taskrunner

PR #4392 was merged to master *after* allocrunnerv2 was branched, so the
client-specific portions must be ported from master to arv2.

burdandrei/nomad,dvusboy/nomad,burdandrei/nomad,hashicorp/nomad,dvusboy/nomad,hashicorp/nomad,burdandrei/nomad,nak3/nomad,hashicorp/nomad,nak3/nomad,nak3/nomad,nak3/nomad,dvusboy/nomad,hashicorp/nomad,nak3/nomad,hashicorp/nomad,burdandrei/nomad,nak3/nomad,nak3/nomad,burdandrei/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad,dvusboy/nomad,hashicorp/nomad