Reduced the runtime of the script test_dropout.
Added the yaml files mnist_fast.yaml and mnist_valid_fast.yaml.
Note: these files should have been included in the previous commit.
From now on, the script runs using the faster settings only if the mode is set to DEBUG_MODE.
Removed outdated documentation
Reverted some of the settings to their previous values.
Merge pull request #1015 from AlassaneNdiaye/reduce_runtime_test_dropout

Reduced the runtime of the script test_dropout.