Provide access to EC2 server tags in Capistrano recipes

[eightbitraptor]

I needed a way of being able to access values for custom tags that were set across machines from within a Capistrano recipe.

It allows you to write something like this

task :run_arbitrary_command do
  on roles(:web) do |server|
    execute :script, server.ec2_tags['tag_name']
  end
end

It does this by storing a reference to the AWS Instance ID when the Capistrano server object is created inside the ec2_role method.

We can use this id later to rebuild the original AWS::EC2::Instance object referring to that server so we can query it for it's tags, which is done by adding a method to the Capistrano Server class so that instances can fetch their own EC2 tags.

[eightbitraptor]

Closing this for now as I'm messed up the branch. Will clean up and re-open.

[andytinycat]

This seems totally reasonable - if you submit another PR I'll merge it.

[eightbitraptor]

thanks. Resubmitted as https://github.com/forward3d/cap-ec2/pull/29