Create exec_dir if it does not exist

[jgaida]

I got the following error trying to create the directory using the JDL prologue:
The job exited because of a working directory error. NFS problem on the cluster?;

[bzizou]

Yes, it is a good idea to create the exec_dit if it does not exist.
Two solutions:

• Using the OAR api by posting an empty file into that directory (e.g. POST /media/exec_dir/dummy_file), but which module can do that?
• Adding an inconditionnal "mkdir -p" at the top of the prologue.
  The second solution is very simple but requires that there's a prologue. For the moment, the prologue is optional. It could be mandatory...

[jgaida]

We can make it optional by adding a "create_dir" option in the JDL