Cycle multiple instances at once

By default aws-ha-release cycles one server at a time. This can take an absurdly long time if you have a lot of active servers, and isn't always necessary.

This PR adds a new option to aws-ha-release (`--num-simultaneous-instances NUM` or `-n NUM`) which lets you cycle multiple servers at once. Instead of specifying an integer you can also say 'MAX' which will spin up as many servers as you have currently running.

Note that if I'm bringing up N servers simultaneously, all N will have to be healthy for the script to continue. If any one of the N are unhealthy once the allotted time is up, then the script will raise an error and exit.

I also changed the executable from `aws-ha-release.rb` to just `aws-ha-release`. The `.rb` should never have been there as I think convention is to drop the extension from an executable.

The `fake_core_data_list.rb` file is mostly copied from `aws-sdk-ruby` itself. I haven't found a good solution for faking out AWS, so for now I'm just recreating the objects.