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
Beginners don't understand the idea of factories, so I doubt referring to factories will help. I always see beginners work only with closures, also because the manual uses them (too) often.