* Add wait timeout settings to receive queue message

* add tests

* add more tests and remove servicebus from coveragerc

* Add transports based on Azure PaaS

This pull request adds two new transport implementations:

- `azurestoragequeues` is implemented on top of Azure Storage
  Queues [1]. This offers a simple but scalable and low-cost PaaS
  transport for Celery users in Azure. The transport is intended to be
  used in conjunction with the Azure Block Blob Storage backend [2].

- `azureservicebus` is implemented on top of Azure Service Bus [3] and
  offers PaaS support for more demanding Celery workloads in Azure. The
  transport is intended to be used in conjunction with the Azure
  CosmosDB backend [4].

This pull request was created together with @ankurokok, @dkisselev,
@evandropaula, @martinpeck and @michaelperel.

[1] https://azure.microsoft.com/en-us/services/storage/queues/
[2] https://github.com/celery/celery/pull/4685
[3] https://azure.microsoft.com/en-us/services/service-bus/
[4] https://github.com/celery/celery/pull/4720

* Exclude Azure transports from code coverage

There is test coverage for the transports but the tests require Azure
credentials to run (passed via environment variables) so codecov doesn't
exercise them.

* Remove env vars to configure transport

* Remove abbreviations