Update docs to mention that Azure default resource group is cloudbridge

[FabioRosado]

This one caught me a bit off guard when I tested the new dev account. I followed the docs on Create a provider and omitted azure_resource_group so cloudbridge was looking for the cloudbridge resource group, which was causing pretty much every test to fail.

If you would like, I'm happy to update this portion of the docs 😄

[nuwang]

Aah, that's bad. Yes, please do. Perhaps we could also link to this section: http://cloudbridge.cloudve.org/en/latest/topics/setup.html