tests/utils/helpers:

Simplified and corrected a util function.
engine:

Fixed the file parsing in `parse_config`. The file dict returned by
this function is now properly keyed by the parameter name (example:
"exposure_file") instead of the input type (example: "exposure").

There are corresponding changes in `create_hazard_calculation` and
`create_risk_calculation`.
engine:

Updated/corrected docstring for `parse_config`.
tests:

Updated tests to run properly with the changes to `engine.parse_config`.
Merge pull request #1246 from larsbutler/file-parsing-fix

File parsing fix

[r=micheles] [f=1214301]