Terraform/OpenStack: Enable usage of an existing router

[qvicksilver]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespaces from that line:

/kind api-change
/kind bug
/kind cleanup
/kind design
/kind documentation
/kind failing-test
/kind feature
/kind flake

What this PR does / why we need it:
It allows the Terraform/OpenStack install to use an existing router
Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Terraform/OpenStack: Support for using existing OpenStack routers has been added, no action needed on current installations. See documentation in contrib/terraform/openstack for instructions on how to use on a new installation.

[holmsten]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: holmsten, qvicksilver

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• contrib/terraform/OWNERS [holmsten]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment