AzureRMR 1.0.0.9000

Significant interface changes

New create_azure_login, get_azure_login and delete_azure_login functions to handle ARM authentication. While directly calling az_rm$new() will still work, it's recommended to use create_azure_login and get_azure_login going forward. Login credentials will be saved and reused for subsequent sessions (see below).
get_azure_token significantly revamped. It now supports four authentication methods for obtaining AAD tokens:
- Client credentials (what you would use with a "web app" registered service principal)
- Authorization code (for a "native" service principal)
- Device code
- With a username and password (resource owner grant)
get_azure_token will now cache AAD tokens and refresh them for subsequent sessions. Tokens are cached in a user-specific configuration directory (unlike httr, which saves them in a special file in the R working directory).
Token acquisition logic will shortly move to a new package, to allow it to be used by other packages independently of the Resource Manager interface.

Don't print empty fields for ARM objects.
Add optional etag field to resource object definition.
Add location argument to az_resource_group$create_resource method, rather than hardcoding it to the resgroup location.
Add wait argument when creating a new resource, similar to deploying a template, since some resources will return before provisioning is complete. Defaults to FALSE for backward compatibility.
Export is_azure_token.
Allow az_resource_group$deploy_template() to work without parameters arg (parameters folded into template itself).
Fix a bug that kept az_resource_group$delete_resource from deleting the resource.