GUACAMOLE-1572: Match environment variables for POSTGRESQL auth plugin doesn't match guacamole.properties variable name

[jbpaux]

Fix issue described in https://issues.apache.org/jira/browse/GUACAMOLE-1572 where environment variables for POSTGRESQL auth plugin doesn't match guacamole.properties variable name

[jbpaux]

I also remarked the user variable for both 3 auth modules is named user in environment variables and username in guacamole.properties, should be updated too. I'll update the documentation when we're ok on all changes here. No sure if it fits in 1.5.0 or not but in case of I've set upstream on 1.5.0.

[necouchman]

@jbpaux This should be against master, it will not go into the 1.5.0 release.

[jbpaux]

If you have time to have a look at it @necouchman (and my question about the username environment variable too) ;)

[jbpaux]

Hi, any feedbacks ? :)

[necouchman]

Overall I think it looks okay to me. @mike-jumper any additional concerns?