Timeout for apps should be duration instead of int to match Helm v3

[wrdls]

Helm v3 changed the --timeout flag from int to time.Duration so it now requires a unit of duration, e.g. 300 becomes 300s or 5m0s.

Since Helmsman v3.x.x only works with Helm versions >=3.0.0 I think it would better if the same change would be applied to Helmsman. The current situation is pretty confusing.

I'm willing to prepare a PR for this if this is acceptable. Obviously this a breaking change.