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
Merge "Add additional help text for openstack user/pass"