Make Argo Workflows trigger action work for any instance

[iainlane]

In this action we make assumptions about where we get the keys from (Vault) and the hostnames of the Argo Workflows instances.

We could make this more generic somehow, maybe taking inspiration from this?

I don't think we're going to work on this from the Grafana side (since the action is good for our purposes already), but if anyone else wants to, that'd be welcome.