Add support for TO_API_VERSION env var in to-access.sh

[rawlinp]

What does this PR (Pull Request) do?

By default, make requests at 2.0 but allow overriding the version when
necessary to deal with older TO servers.

• This PR is not related to any Issue

Which Traffic Control components are affected by this PR?

• CDN in a Box

What is the best way to verify this PR?

With TO_URL, TO_USER, and TO_PASSWORD vars set appropriately:
. to-access.sh; to-auth -> verify that the login request was made to TO at API version 2.0.
TO_API_VERSION=1.4; . to-access.sh; to-auth -> verify that the login request was made to TO at API version 1.4.

Start up a Docker CIAB instance, make sure it all starts up properly as it depends on the to-access.sh script.

The following criteria are ALL met by this PR

• Starting up CIAB tests this script pretty well -- doesn't have its own specific tests
• The to-access.sh script doesn't have docs
• A changelog entry would be a little overkill for this undocumented script
• This PR includes any and all required license headers
• This PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[zrhoffman]

Do you think TO_API_VERSION should be in variables.env?

[rawlinp]

I don't think it would be required there. In general CIAB should just take the default version set in the script itself -- only users sourcing the script directly for non-CIAB purposes would care to set a specific version.

[mhoppa]

works in CDN box, works manually testing setting the API version to different versions