EC2 Backend Specs

Creating the Pull Request very early. This is definitely not to be merged now. But is should make it easier for maintainers to check out the new code in case I need help (which I actually do need).
Absolute paths, please! You can use:
```ruby
Rails.root
```
to reference the main project directory.
```ruby
YAML.load_file("#{Rails.root}/spec/lib/backends/ec2_stubs/instance_statuses_stub.yaml")
```
It's also a good practice to use `File.join`:
```ruby
YAML.load_file(File.join(Rails.root, 'spec', 'lib', 'backends', 'ec2_stubs', 'instance_statuses_stub.yaml'))
```