* vendor updates
- updating to v21.3.0 of github.com/Azure/azure-sdk-for-go
- updating to v10.15.4 of github.com/Azure/go-autorest
- vendoring github.com/hashicorp/go-azure-helpers @ 0.1.1
* backend/azurerm: refactoring to use the new auth package
- refactoring the backend to use a shared client via the new auth package
- adding tests covering both Service Principal and Access Key auth
- support for authenticating using a proxy
- rewriting the backend documentation to include examples of both authentication types
* switching to use the build-in logging function
* documenting it's also possible to retrieve the access key from an env var
This work was done against APIs that were already changed in the branch
before work began, and so it doesn't apply to the v0.12 development work.
To allow v0.12 to merge down to master, we'll revert this work out for now
and then re-introduce equivalent functionality in later commits that works
against the new APIs.
Tom Harvey