Default project configuration

Now we set configuration settings via system env. variables - which is not always convenient for users which just want to try it out. We need to have a default configuration which can be overwritten on demand by environment variables and properties file.
We need to get rid of Maven resource filtering that is used right now to pull env variables and rely on Spring abilities. So what we need is:

• All properties (in files and in env vars) should have same name: JTALKS_DB_USER & JTALKS_DB_PASSWORD should replace the respective property in the file, also JTALKS_DB_URL should be possible to set via env vars
• The default values should be used in spring configuration:
  • JTALKS_DB_USER=root
  • JTALKS_DB_PASSWORD=root
  • JTALKS_DB_URL=jdbc:mysql://localhost/p_poulpe?characterEncoding=UTF-8

This is the only configuration we need to be able to specify both in properties file and env vars and in the same time have default values. Other should reside only in file.
After the implementing the task, please update the Installation Guide with information about defaults and how to override them.