Provide the user password through an environment variable

[bcogrel]

• Try for most (if not all) Docker images to create the user at container initialization time using the provided password
• Change the properties files in the ontop-docker-tests module and inject a default value in the main pom.xml file of Ontop
• Adapt the Gitlab and Travis scripts to use an environment variable whose value is provided by the environment of the script, not the script itself
• Move the default passwords to a .env.example file
• Update the README and propose the users to do cp .env.example .env.