Flexible PostgreSQL configuration

• Make the PostgreSQL plugin configurable for alternative docker images that use different users, paths or startup options
• Provide the database administrator password as an environment variable
Flexible MariaDB configuration

• Make the MariaDB plugin configurable for alternative docker images that use different users, paths or startup options
• Provide the database administrator password as an environment variable
Flexible MongoDB configuration

• Make the MongoDB plugin configurable for alternative docker images that use different users, paths or startup options
• Provide the database administrator password as an environment variable
Flexible Redis configuration

• Make the Redis plugin configurable for alternative docker images that use paths or startup options
DRY up the database plugins

• Remove the option to provide the database admin user password in a ENV variable.
  Linked containers get access to docker provided environment variables, this leaks
  the password into applications that shouldn't have access to the password.
Fix indentation
Merge pull request #38 from yabawock/feature/configuration-options

Enhanced database plugin configuration