Add additional help text for openstack user/pass

Update the openstack example config file with some additional help
text about the username and password.  The first time I ran this it
took me a little bit to figure out the right password to use.  I
didn't realize I needed to go generate a password in my gerrit
settings.  Hopefully this note will avoid confusion by someone else in
the future.

Change-Id: If93ec938e48f624aff4741b19d95e03abb8c8a9c

examples/openstack-gertty.yaml

@@ -6,7 +6,11 @@
 servers:
   - name: openstack
     url: https://review.openstack.org/
+    # Your gerrit username.
     username: CHANGEME
+    # Your HTTP Password for gerrit. Go to the "HTTP Password" section in your
+    # account settings to generate/retrieve this password.  It is *not* your
+    # launchpad password.
     password: CHANGEME
     git-root: ~/git/