Set stall timeout from swell launch

[mranst]

@rtodling This pr adds the flag '-t` to the swell launch command to dynamically set the cylc stall timeout.

swell launch <experiment> -b -t PT30S

The launch timeout is usually set to an hour by default, or to the value under ~/.cylc/flow/global.cylc. This allows the user to set it manually, intended mostly for the purpose of throwing an error immediately after the suite fails. The implementation here is not ideal, but this will be made simpler in #656.

[shiklomanov-an]

The variable = "unset" thing is an anti-pattern, and cuts off the ability to set this directly via environment variable (which seems useful to preserve). But otherwise, this looks good — happy to approve once that's resolved.

[shiklomanov-an]

Two things:

1. Per my comment above, setting the variable to "unset" is an anti-pattern. I would just leave the variable literally unset.

        if self.cylc_timeout is not None:
            os.environ['SWELL_CYLC_TIMEOUT'] = self.cylc_timeout
        # no else

2. Do you want to give users the ability to control this directly via environment variable? Might come in handy for certain edge cases. If so, consider leaving any user-specified values of SELF_CYLC_TIMEOUT unchanged.

        if self.cylc_timeout and 'SELF_CYLC_TIMEOUT' not in os.environ:

[mranst]

Fixed, thanks. As for 2, the way it is currently a user would have to go out of their way to specify -t, so I think it would be best to allow that to override the environment variable, so the hierarchy of control would be (in ascending order)

1. Cylc default
2. Value in global.cylc
3. User-set environment variable
4. User-specified value on command line