OGM-408 Moving MongoDB property constants to OgmProperties and MongoDBProperties
OGM-408 Using common properties for configuring CouchDB
OGM-408 Making CouchDB's association storage option a generic document store option
OGM-408 Using @AssociationStorage option and property from core for MongoDB
OGM-408 Unifying access to association storage configuration property
OGM-408 Moving common configuration constants to OgmProperties
OGM-408 Unifying retrieval of common properties

* Extending ConfigurationPropertyReader to read simple configuration properties
* Establishing DocumentStoreConfiguration to access properties common to MongoDB and CouchDB