Unable to set lambda to use unreserved concurrency in chalice config.

Setting this
`"reserved_concurrency": null,`

results in the lambda applying the _reserved_concurrency_ setting at the project level when applying lambda specific configurations.

If I have this set to _1_ at the project level, setting to _null_ for a specific lambda results in 1 as the actual setting. It appears there is no way to specify "use unreserved concurrency" when configuring using chalice _config.json_.

Trace how Chalice reads project-level and Lambda-specific reserved_concurrency settings from config.json, starting with the configuration merge and Lambda deployment paths. Confirm the behavior with a project value of 1 and a function value of null; done means the function uses unreserved concurrency instead of inheriting 1.

Written by the indexing model from the issue text.