Add support for remote port fowarding for SSH connections used by Provisioners

[peteromoon]

During the bootstrapping phase the instances you are provisioning might not yet have
direct network connectivity to resources required for provisioning. If the node used
for provisioning has the required access, then by using SSH remote port forwarding
we can give the node being provisioned network access to the required resources.

This PR adds that capability for provisioners when using an SSH connection.

A new parameter is added to connection type for SSH connections only. The
parameter is used to specify a list of ports forward from host being
provisioned to the local side of the SSH connection (the side running Terraform).

The remote port forwarding is established when the SSH connection is made to
the host being provisioned and it torn down when the SSH connection terminates.

Failure to establish the remote port forwarding does not abort the provisioner,
but does generate a log warning message.

[apparentlymart]

Thanks for working on this, @peteromoon!

We have this older issue #8367 where we discussed a more general solution to this problem that would work for other resource operations (e.g. retrieving data from a Consul server in the remote network), but your approach here has the advantage of relative simplicity by focusing on a specific situation.

I'm feeling a little ambivalent about merging this and then later having the more generalized solution which could then prove confusing. I'd like to mull this over for a little while, rather than rushing in to merging this.

[teamterraform]

Hi folks! Now that Terraform 0.12 is released, we are undertaking a general GitHub issue/PR cleanup.

This PR touches code that has changed quite a bit since it was originally opened, and in this case it . I am going to close this outdated PR, but please do not hesitate to open a new PR or issue in the future, and thank you!

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.