remove timeout from remote-exec command context

The timeout for the remote command was taken from the wrong config
field, and the connection timeout was being used which is 5 min. Any
remote command taking more than 5 min would be terminated by
disconnecting the communicator. Remove the timeout from the context, and
rely on the global timeout provided by terraform.

There was no way to get the error from the communicator previously, so
the broken connection was silently ignored and the provisioner returned
successfully. Now we can use the new cmd.Err() method to retrieve any
errors encountered during execution.

builtin/provisioners/remote-exec/resource_provisioner.go

@@ -156,10 +156,6 @@ func runScripts(
 	o terraform.UIOutput,
 	comm communicator.Communicator,
 	scripts []io.ReadCloser) error {
-	// Wrap out context in a cancelation function that we use to
-	// kill the connection.
-	ctx, cancelFunc := context.WithTimeout(ctx, comm.Timeout())
-	defer cancelFunc()
 
 	// Wait for the context to end and then disconnect
 	go func() {
@@ -200,10 +196,14 @@ func runScripts(
 		if err := comm.Start(cmd); err != nil {
 			return fmt.Errorf("Error starting script: %v", err)
 		}
-
 		cmd.Wait()
-		if cmd.ExitStatus != 0 {
-			err = fmt.Errorf("Script exited with non-zero exit status: %d", cmd.ExitStatus)
+
+		if err := cmd.Err(); err != nil {
+			return fmt.Errorf("Remote command exited with error: %s", err)
+		}
+
+		if cmd.ExitStatus() != 0 {
+			err = fmt.Errorf("Script exited with non-zero exit status: %d", cmd.ExitStatus())
 		}
 
 		// Upload a blank follow up file in the same path to prevent residual