plugin/consul-upstreams: tag parsing

We should add the ability to include tags for the `consul-upstreams` plugin.

Specifically, this should allow the following:
- `protocol:http-internal|http-public|https-public|https-internal`
- `hostname:foo`
- `prefix:foo`
- `timeout:foo`

We should cascade tags properly by looking for them on the service and then overriding them on an instance by instance level.
