Migrate away from RequestSpotInstances API

[chadlwilson]

Currently the plugin seems to rely upon this API to get spot instances. According to https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_RequestSpotInstances.html this API is deprecated.

I'm not sure what the best alternative is for this plugin from https://docs.aws.amazon.com/en_us/AWSEC2/latest/UserGuide/spot-best-practices.html however at the same time it'd be good to move away from the code this plugin uses to get spot instances

• avoid manual code-based selection of target subnet based on least instances (this leads to trying to create instances in an AZ that has a capacity problem repeatedly, which is not great)
  • plugin should support just giving it a few subnets and let it create instances based on capacity
• allow different capacity strategies, preferably delegates to AWS (maybe via CreateFleet)