Add ssh module for running commands over ssh

[yorinasub17]

This ports over the ssh module from terratest so that we can use it in our CLI tooling to SSH to instances. An example use case is SSH-ing to a deployed jenkins instance in the Reference Architecture to retrieve the admin token for configuring Jenkins over the API.

Note that this implementation has several changes over the terratest version:

• The terratest functions only supported a single hop for jump hosts. This implementation supports arbitrary hops by treating the JumpHost parameter as a linked list.
• Connection state tracking is simplified to a single Stack data structure that tracks all the items that need to be closed.
• HostKeyCallback is now configurable at the top level Host data structure.

[yorinasub17]

Example use case: https://github.com/gruntwork-io/terraform-aws-architecture-catalog/pull/155

[yorinasub17]

UPDATE:

• Better error handling so that the sshAgent can be stopped if there is an error loading keys to it: 72c9d76
• Add support for adding non-RSA private PEM keys to ssh agent: 77f1a09

[yorinasub17]

Thanks for review! Merging this in now!