Improve hyperparameter organization about evalml HOT 3 CLOSED

I want to add a couple more points here which are related to this issue. These just came up in @angela97lin's catboost PR:

1. We should allow the sub-library to specify intelligent defaults whenever possible. Example: catboost's loss_function and bootstrap_type are both chosen by the library based on certain heuristics. Unless we have a good reason to override those heuristics, we should make use of them, because otherwise we risk negatively impacting performance.
2. Question: do we need a distinction between a) hyperparameters which automl will search over and b) the list of hyperparameters supported by a particular pipeline? This would be required if we wanted to have automl tuners operate on different sets of hyperparameters, so I guess the real question is whether or not we want to support that. Perhaps that should simply be filed as a future feature.

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@jeremyliweishih is this a duplicate of #359 ?

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Closing in favor of #359 and the other tickets we've got tracking hyperparameter improvements

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