Abstract out concepts of DataSourceType and DatabaseConfigInitializer.

Encapsulate data source specific logic into subclasses of DataSourceType
and DatabaseConfigInitializer.  DataSourceType and
DatabaseConfigInitializer are separated because we need them at
different stages (init vs runtime) and they also need to exist in
different projects with different dependencies.

This is a first step toward providing support for different database
platforms as plugins.

Change-Id: I01ea666ef7f682d6137f0e64005d979dd706f67b
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
Signed-off-by: Sasa Zivkov <sasa.zivkov@sap.com>