Add credentials of Azure reseller customers for billing only

For Azure partners (CSP account), a customer can display a billing dashboard without using Azure's compute features in the platform.

There are two ways to add Azure customer accounts for billing only:

1. Add the tenant ID so Abiquo will automatically onboard the customer and all their subscriptions into a single tenant. See https://abiquo.atlassian.net/wiki/spaces/doc/pages/514785296

2. Add a subscription to a standard tenant. For this configuration we recommend that you use a main keynode tenant for the non-subscription costs. See https://abiquo.atlassian.net/wiki/spaces/doc/pages/516128903.

For full details of how to create a tenant hierarchy for CSP resellers, see https://abiquo.atlassian.net/wiki/spaces/doc/pages/443056256.