Update GT.M installation instructions following best practices

Update the user and group used for GT.M installation.

Change the directory to follow GT.M debian packaging defaults.

Create VistA instance using an instance owner user.

Instruct users howto create a profile script for VistA using sane
defaults.

Instruct users how to create a db.gde script for VistA using sane
defaults.

Change-Id: Ibd8cddc03d6d54762e7bd2361db23ae7dc955d35
Suggested-by: sam.habiel@gmail.com