allocs: Add nomad alloc stop

This adds a `nomad alloc stop` command that can be used to stop and
force migrate an allocation to a different node.

This is built on top of the AllocUpdateDesiredTransitionRequest and
explicitly limits the scope of access to that transition to expose it
under the alloc-lifecycle ACL.

The API returns the follow up eval that can be used as part of
monitoring in the CLI or parsed and used in an external tool.

api/allocations.go

@@ -89,6 +89,20 @@ func (a *Allocations) Restart(alloc *Allocation, taskName string, q *QueryOption
 	return err
 }
 
+func (a *Allocations) Stop(alloc *Allocation, q *QueryOptions) (*AllocStopResponse, error) {
+	var resp AllocStopResponse
+	_, err := a.client.putQuery("/v1/allocation/"+alloc.ID+"/stop", nil, &resp, q)
+	return &resp, err
+}
+
+// AllocStopResponse is the response to an `AllocStopRequest`
+type AllocStopResponse struct {
+	// EvalID is the id of the follow up evalution for the rescheduled alloc.
+	EvalID string
+
+	WriteMeta
+}
+
 // Allocation is used for serialization of allocations.
 type Allocation struct {
 	ID                    string