Fix doc for adding sudo privileges to stack user

Writing NOPASSWD directive into /etc/sudoers was throwing
permission denied errors. This commit writes the directive
to the /etc/sudoers.d/stack file instead.

Closes-Bug: #1981541
Change-Id: If30f01aa5f3a33dda79ff4a6892116511c8e1542

Author: kopecmartin

doc/source/guides/multinode-lab.rst

@@ -89,7 +89,7 @@ password:
 
 ::
 
-    echo "stack ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers
+    echo "stack ALL=(ALL) NOPASSWD: ALL" | sudo tee /etc/sudoers.d/stack
 
 From here on use the ``stack`` user. **Logout** and **login** as the
 ``stack`` user.

doc/source/guides/single-machine.rst

@@ -63,7 +63,7 @@ to have sudo privileges:
 .. code-block:: console
 
     $ apt-get install sudo -y || yum install -y sudo
-    $ echo "stack ALL=(ALL) NOPASSWD: ALL" >> /etc/sudoers
+    $ echo "stack ALL=(ALL) NOPASSWD: ALL" | sudo tee /etc/sudoers.d/stack
 
 .. note:: On some systems you may need to use ``sudo visudo``.