Allow distinct_host to have L/RTarget set

This PR removes validation that could break job backwards compatibility.
The targets are ignored so there is no side effects.

Fixes #3130

nomad/structs/structs.go

@@ -3973,12 +3973,6 @@ func (c *Constraint) Validate() error {
 	switch c.Operand {
 	case ConstraintDistinctHosts:
 		requireLtarget = false
-		if c.RTarget != "" {
-			mErr.Errors = append(mErr.Errors, fmt.Errorf("Distinct hosts constraint doesn't allow RTarget. Got %q", c.RTarget))
-		}
-		if c.LTarget != "" {
-			mErr.Errors = append(mErr.Errors, fmt.Errorf("Distinct hosts constraint doesn't allow LTarget. Got %q", c.LTarget))
-		}
 	case ConstraintSetContains:
 		if c.RTarget == "" {
 			mErr.Errors = append(mErr.Errors, fmt.Errorf("Set contains constraint requires an RTarget"))

nomad/structs/structs_test.go

@@ -1353,14 +1353,10 @@ func TestConstraint_Validate(t *testing.T) {
 
 	// Perform distinct_hosts validation
 	c.Operand = ConstraintDistinctHosts
-	c.RTarget = "foo"
-	err = c.Validate()
-	mErr = err.(*multierror.Error)
-	if !strings.Contains(mErr.Errors[0].Error(), "doesn't allow RTarget") {
-		t.Fatalf("err: %s", err)
-	}
-	if !strings.Contains(mErr.Errors[1].Error(), "doesn't allow LTarget") {
-		t.Fatalf("err: %s", err)
+	c.LTarget = ""
+	c.RTarget = ""
+	if err := c.Validate(); err != nil {
+		t.Fatalf("expected valid constraint: %v", err)
 	}
 
 	// Perform set_contains validation