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
Thought about that as well. The whole idea with these constants is to help beginners to get into the ZF2. Pros won't really need it. So if you are a beginner what would make more sense to you.

Variant A)

```
use Zend\Db\Adapter\Adapter;

return array(
    Adapter::CONFIG => array(),
);
```

Variant B)

```
use Zend\Db\Adapter\AdapterServiceFactory;

return array(
    AdapterServiceFactory::CONFIG => array(),
);
```

Thoughts?