FSL Melodic: specify tool config for fsl_sub

[glatard]

Currently, fsl_sub doesn't specify a tool config when it submits tasks to CBRAIN. The Bourreau picks the first tool_config it finds, which has several issues:

• the new task may be submitted to a different cluster;
• the new task may be submitted with a different version of FSL.
  So, at the moment, FSL melodic (group analyses) only work when there is only a single tool config for fsl_sub, with the same version as FSL melodic, and on the same cluster.

The solution to allow for more fsl_sub tool_configs is to:

• choose the proper tool_config_id in fsl_melodic (bourreau-side). The tool config should be on the same bourreau, with the same FSL version (fsl_sub currently use different versions than FSL, but this should be fixed in fsl_sub's Boutiques descriptor).
• pass this tool config id to fsl_sub, e.g. through an environment variable.
• make fsl_sub write the tool_config_id in its job submission to CBRAIN.