Correctly quote shell params when they are passed off

I found this little bug today caused by a tiny little issue in the .sh files. They say in the help that I can pass in a short description via the `-sd` option. I would normally expect descriptions to use space, but they break the shell script since failing to quote `$@` is causing double word expansion.

Issuing this command will reproduce the issue which this PR solves:

    $ ./bin/gatling.sh -sd "test test"
    GATLING_HOME is set to /home/<user>/gatling
    Error: Unknown argument 'test'
    
    Usage: gatling [options] 
    
    ...