New fixtures with correct keys and metadata for pre-installed maps

This PR adds new fixtures with correct keys and metadata for pre-installed maps.
It fixes several bugs:
- wrong or empty thumbnails
- wrong cross references between layers and maps
- wrong names on the layers
- legend in the pre-created maps

and other minor bugs.

https://bugs.launchpad.net/oq-platform/+bug/1375758
```common_``` instead of ```initial_data_``` prefix to be more consistent.