Can we consolidate the common pipeline, agency pipeline, and azure-vars

[thekaveman]

Following up from #292 / #346

Our ideal state:

• A single pipeline file that defines all triggers, stages, etc., used by all deployments/agencies
• An azure-vars.yml file per-agency that specifies agency-specific variables like the name of the Terraform backend resource group, or the name of the app service
• A runtime parameter / variable / something that says which agency the run is happening for
• Use the ☝️ parameter to load variables from the correct agency directory azure-vars.yml file

We noted during a working session that this seems to not be possible with the current syntax and available parameter/variable definition features.

Maybe do this as part of #294 if we end up going that direction.

[thekaveman]

This is working for our two current agencies and we don't plan to add more.