Support multiple instances of DNS Providers

[jsblake]

Is your feature request related to a problem? Please describe.

We have many, many DNS zones in our Azure tenant, and they are in different subscriptions depending on their use, team ownership, provenance, etc. The Azure DNS provider configuration only seemingly accepts one subscription ID, meaning for each instance of keyvault-acmebot we can only issue certs against zones in a single subscription

This causes two issues:

1. We need multiple instances of keyvault-acmebot to support all of our DNS Zones
2. We cannot issue single certificates that have Subject Alternative Names in two or more zones that are in these other subscriptions

(this is related to issue #666 regarding multiple keyvaults, in that in combination these two features would allow us to manage all of our certs across our Azure tenant with one instance of keyvault-acmebot)

Describe the solution you'd like
Allow multiple instances of configuration for any given DNS provider as much as possible (understanding that for some providers this may not make sense or be possible)--but at least allow access to DNS zones in more than one subscription in Azure. Note I am not asking for access to zones in other tenants

Describe alternatives you've considered
Currently the only alternative is to run multiple instances of keyvault-acmebot and for certs that cross these multiple zones, issue with other tools like CertifyTheWeb or purchase from a traditional CA

[pwhitney5524]

This feature would be great!

We have hundreds of clients for which we do not manage DNS and would need to be able to add multiple DNS providers with multiple API users for each in order to fully automate certificate management using this tool.