Unmount task directories when alloc is terminal
Unmount task directories when alloc is terminal

This PR unmounts directories from tasks when the alloc is terminal
rather than when it is garbage collected.

/cc @angrycub

dvusboy/nomad,Ashald/nomad,hashicorp/nomad,burdandrei/nomad,nak3/nomad,nak3/nomad,nak3/nomad,nak3/nomad,hashicorp/nomad,frundh/nomad,frundh/nomad,Ashald/nomad,nak3/nomad,hashicorp/nomad,dvusboy/nomad,nak3/nomad,burdandrei/nomad,dvusboy/nomad,Ashald/nomad,Ashald/nomad,frundh/nomad,Ashald/nomad,hashicorp/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad,burdandrei/nomad,frundh/nomad,frundh/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,Ashald/nomad,frundh/nomad,frundh/nomad,nak3/nomad,hashicorp/nomad,hashicorp/nomad,burdandrei/nomad,Ashald/nomad,dvusboy/nomad