Default for start_timeout is not respected for slurm launcher

The default for start_timeout is -1 in the SlurmLauncher class, so that client waits indefinitely for Fluent to launch. But, it comes as 60 at runtime within SlurmLauncher. To reproduce outside of the slurm environment:
1. Add a `pdb.set_trace()` or a breakpoint at the beginning of `SlurmLauncher.__init__`.
2. Execute `pyfluent.launch_fluent(scheduler_options={"scheduler": "slurm"})`, see the value of `start_timeout` is 60 in the debugger.