Global variables are currently ignored by experiment execution

It seems that global variables are currently ignored by experiment execution, while they are displayed properly on the experiment page. I don't know if the problem is at the beat.web or beat.core level.

To reproduce the problem:

1. Create a simple Eigenface experiment and run it.
2. Fork it and update the 'global' number of pca components parameter.
3. After pressing go on the forked experiment, it will complete very fast and leads to the exact same results.