Added support for modifying grid configuration option throwOnCapabili…

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[MicahZoltu]

Is there anything holding up this PR?

I just spent half a day trying to figure out how to make it so I can run tests via docker-compose and found that the tests will often run before the nodes are all finished connecting to the hub. With throwOnCapabilityNotPresent set to true the hub fails the request from the test driver setup because it doesn't have any capabilities yet. I believe setting this would resolve the problem and make it so the requests from my tests will queue up until the necessary node is fully connected.

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@MicahZoltu yes, this PR fixes this exact issue you described. Until it's merged, you can use a workaround described in #332.

[MicahZoltu]

I ended up solving the problem myself by building my own docker image. I would have preferred to use an environment variable as it would have been a much cleaner solution, but this got things working for now.
https://github.com/Zoltu/bags-client/blob/master/functional-tests/Dockerfile-selenium-hub