Update environment variables and documentation

[notlee]

Review environment variables and correct documentation as required. For example it doesn't look like FASTLY_PURGE_API_KEY is used; this todo is in the readmen and is 5 years old:

TODO: The options below are required at the moment, but are duplicates of other options above. This will be addressed once all services are using Origami Makefile.

    FASTLY_API_KEY: The Fastly API key to use when purging assets. If not set, purge endpoints are not registered. This should be the same value as FASTLY_PURGE_API_KEY
    FASTLY_SERVICE_ID: The Fastly service to purge assets from
    API_KEY: The API key to use when purging assets. If not set, endpoints which require an API key are not registered. This should be the same value as PURGE_API_KEY

We should also removed unused environment variables from Vault.