* When support for service tags was added, the only way we had to parse
and dedup a list of strings also forced them to be lowercase. Now there's
another helper func that doesn't smash the case so use that instead.
* update Consul 'service_tag' documentation to include case sensitivity
* added upgrade guide for 1.15
* test for service tags
Co-authored-by: Nick Cabatoff <ncabatoff@hashicorp.com>
Co-authored-by: Peter Wilson <peter.wilson@hashicorp.com>
