Bugfix: Allow use of container level SAS/SAT tokens with Azure backend

When using a SAS/SAT token for authentication with Azure, restic was expecting
the provided token to be generated at the account level, granting permissions
to the storage account and all its containers. This caused an error that did
not allow tokens that were generated at the container level to be used to 
initalize a repository.
Restic now allows SAS/SAT tokens that were generated at the account or 
container level to be used to initalize a repository.

https://github.com/restic/restic/issues/4004
https://github.com/restic/restic/pull/5093