Add support for multi-tenant authentication

[jhendrixMSFT]

This is a hard requirement for TF support.

[magodo]

Adding more context:

Currently, AzureRM provider makes use of auxiliary tenant to allow users to provision resource crossing tenants (e.g. cross tenant vnet peering).

The reason why the AzureRM provider works is because it depends on Go SDK Track2, where it can construct a multi-tenant authorizer for service principal either via adal.NewMultiTenantServicePrincipalToken, or via getting the token from azcli (and convert it into SP token).