[module/ec2] Use tags for idempotence.

Use tags to achieve idempotence for ec2 instances.

This is a better way to do achieve idempotence for ec2 instances. Using "client-token" is a hack, since terminating an instance with a given ID will prevent us from starting a new one with the same ID.

I think that if this pull request is accepted, the code using client-token i.e. the "id" params should be removed.