Allow obtaining a service without specifying its name.

In other words, instead of:

```swift
if let psqlService = ConfigurationManager.getPostgreSQLService("MyPostgreSQLService") {}
```

You can simply do:

```swift
if let psqlService = ConfigurationManager.getPostgreSQLService() {}
```

The rationale is that the majority of projects will only have a single database of that given type in the application that has been configured. So we should have this option available to them.