Add configuration for MongoDB

application.yml contains the placeholder for database information. In order to pass those information from command line execute maven command with the parameter db.uri, db.username, and db.password.

For example:
mvn clean package -Ddb.uri=mongodb://localhost/runnersMY -Ddb.username=root -Ddb.password=root
Initial User implementation

This is not finalised. The main intention is to validate the configuration for MongoDB in application.yml