Cost Mgmt Export jobs - allow for multiple deployments within a single tenant

[wozzer72]

The export function and its dependencies deploy into a given "subscription". Can deploy multiple times within a single azure tenant with separation.

Except for the Cost Management exports. Cost Mgmt is a singular (global) service within the tenant - across all subscriptions.

Therefore, it is necessary to support being able to create separate cost mgmt export jobs (both the daily export - created in terraform and the backfill exports - created by the deployed function).l

[wozzer72]

Introducing module variable cost_mgmt_suffix; default to empty so no suffix. But when set to any value, it will introduce the name into the cost export task name - as shown above for value of "dev".

[wozzer72]

terraform apply following introduction of suffix.

[wozzer72]

And now the backfill with suffix following forced run on "backfill timer" task on the functionapp:

[wozzer72]

Pushed back a fix to better handle empty "COST_MGMT_SUFFIX" in the python code; initially, the method to get env var threw an exception if empty - rendering the functionapp useless. But then, os.environ.get returns None rather than empty string if the env var is defined but empty.