It sometimes happend that buildout unistall/install the section with this recipe,
which remore storage_path file and recreate it. In most of cases this is not wanted
because the recipe is used to generate password, so that password will change when this
happend.
To prevent buildout to remove storage_path, I introduced a new option 'create-once'
which, when set to True (the default value) will not let buildout play with storage_path file
by returning None in install recipe. Also install won't remove the file if the size is not 0.

/reviewed-on nexedi/slapos!129