Rename prepare_script option to prepare_command

To make the `prepare_script` configuration option more consistent with
the `worker_command` option, rename it to `prepare_command`.

Change-Id: I868afd1f3c4775fa63e9ad5fc872880d4d10e74e
Reviewed-on: https://gerrit.causes.com/33871
Tested-by: Shane da Silva <shane@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>