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
is this really required here? Since TreeRouteStack extends SimpleRouteStack and its present in SRS, it's not really required here.