Add billing UUID to PO

[fereira]

• adds a map containing billing address as a lookup table
• uses billTo environment variable as a key to get billing address UUID
• validates that billTo and other required env vars have been set

Resolves #26

[cappadona]

As is, this will not add a billing address to POs when deployed. We need to add the billTo environment variable to the .env file that is currently stored in an S3 bucket. I will follow up in Slack.

I also have a couple suggestions for refining the API call when retrieving the tenant addresses and the ensuing logic when building the lookup map.

[cappadona]

@fereira I've updated the .env in the S3 bucket so feel free to merge at your leisure.