Support constraints on job instances (probably by making `instance_definition` configurable)

[mboldt]

We have a tile that requires the number of instances of a job to be exactly 1. The instance count is currently hardcoded to be configurable in tile-generator https://github.com/cf-platform-eng/tile-generator/blob/bef5bab4a1f2d61973d9cc92a5fdcfc5d7c8f60c/tile_generator/tile_metadata.py#L594-L606

We should probably just expose the instance_definintion as something the tile author can set.

[cf-gitbot]

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