Feature/#6777 service manager constants

Added constants for all service manages created by the ModuleManagerFactory. Also added configuration keys for database adapters, router and view manager since they are also used a lot in the docs.

Corresponds to https://github.com/zendframework/zf2/issues/6777
They are coupled with the factories using them, so I do not see how moving them to their own interfaces would provide de-coupling :-\