Allow env-set to handle deprecated attributes

Fixes: https://bugs.launchpad.net/bugs/1387953

Customers will not be hitting this issue but the CI scripts do because they set tools-metadata-url during the course of CI testing.

When set-env is used, you can now pass in deprecated attributes and they will be converted to the new values. The code used to initially parse env config is shared.

As a drive by, convert some more config string literals to constants.