create dbvis_add_db_admin.rb

Dbvisulaizer offers a command line functionality to execute SQL pre-configured databases (With GUI).
The remote database can be accessed from the command line without the need to authenticate.
The module abuses this functionality to create an administrator in the database if DB user rights allow it.
You should be doing string interpolation, or string formatting, and not string concatenation. Please see:
https://github.com/bbatsov/ruby-style-guide

Note that we do want people to stick to the Ruby Style Guide above, which is already mentioned in CONTRIBUTING.md:
https://github.com/rapid7/metasploit-framework/blob/master/CONTRIBUTING.md