Change INSTANCE_FILENAME to INSTANCE_NAME

[nizomovs]

The @-Variable INSTANCE_FILENAME could only be used in the args property of generator instances or for instances that are installed from a URL (see this test). With #172 the INSTANCE_FILENAME variable can also be used in i.e. the extra_args property of an experiment.

However, the variable name INSTANCE_FILENAME is misleading, as it is not the file name but rather the name of the instance and I propose changing it to INSTANCE_NAME.