Create /opt/stack and make it home directory

single-machine.rst and index.rst

Before this, one had to create /opt/stack
manually and chown it to the stack user.
Now it is created when the user is created.

This is the same way the multi-node guide
handles it. A stack group is created too.

Change-Id: I5363d81c8fb38796f565cc6ebf6ab2dee2673989
Closes-Bug: #1673787
Closes-Bug: #1671409
diff --git a/doc/source/index.rst b/doc/source/index.rst
index edd6595..f8d5008 100644
--- a/doc/source/index.rst
+++ b/doc/source/index.rst
@@ -56,7 +56,7 @@
 
 ::
 
-   $ sudo adduser stack
+   $ sudo useradd -s /bin/bash -d /opt/stack -m stack
 
 Since this user will be making many changes to your system, it should
 have sudo privileges: