Improve Microprofile-Config examples

[cchet]

• Fix Spec links, they point to wrong spec in some cases.
• Fix wording in Code where it states "microprofile-properties.properties" to "microprofile-config.properties"
• ConfigSource with DB.
  WE MUST REMOVE the config-source after the example, otherwise the server will face performance issues when the other MicroProfiles are added to the classpath.
  Each config key not defined in a higher config-source will cause an search in the DB.
  Each search will cause a connection-established -> select query -> connection-closed !!!!!!!!!!!!!!!
• Add some samples for real world scenaries. Where would we use system properties in a microservice environment? When custom sources?