Touchdown

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- Core: Various serialization fixes regarding detection of 'pending' resources.

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- Provisioner: Provisioner explicitly does not participate in destroy goals.

- AWS EC2: Wait for EC2 instance to be properly terminated

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- AWS Events: Clean up targets before trying to delete rules.

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- AWS EC2: Support `user_data` for ec2 instances.

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- AWS EBS: Fix validation for volume_attachments. Certain operations expected
it to have a name, which is not the case.

- SSH: Fix portforward cmd line entry point, which was incorrectly parsing port
specifications.

- Core: Some graph walking helpers were added to help testing.

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- AWS STS: Add `touchdown get-credentials ROLENAME`. This allows touchdown to
generate some temporary credentials and set them in the environment. The goal
of this feature is that you will be able to do this:

$ eval `touchdown get-credentials ROLE`
$ (deployment) awscli s3 sync ....

But for now the progress bar is also on stdout which stops this working.

- AWS EC2: Add support for attaching EBS Volumes to EC2 Instances.

- AWS EC2: Touchdown now waits after creating a Volume before trying to use it
with other resources.

- SSH: Basic portforwarding rules can now be attached to SSH connections:

ssh_connection.add_port_forward(
name='db',
hostname='localhost',
port=5432,
)

This will allow you to connect to port 5432 on localhost (relative to the
machine you have SSH'd to) with:

touchdown portfwd db=8093

(In this case, port 8093 on your local machine will forward to port 5432 on
the remote machine).