Make Terraform exec timeout configurable

[radeksimko]

This PR raises the default timeout from 10s to 30s to reflect the original timeout may not be sufficient for a combination of a few providers with large schemas used in the same config and slower disk I/O.

It seems pragmatic to accommodate such cases, but slow I/O is painful in general and configuration gets more fragile with more providers you add (e.g. because one API may be down, which makes applies stop half-way through etc.), so these users will suffer either way and this patch is not meant to solve all their problems, just to help them suffer less.

[paultyng]

LGTM, we discussed the ctx forking issue internally, and something we can address down the road once things are more stable.

[ghost]

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