azureclient: stop validating credentials when creating the client

different operations in Azure require different sets of credentials.
some operations for example only need a storage account key to
authenticate with Azure API.

since we cannot predict which operation the caller will need, we cannot
validate auth related options on client instantiation time.

this didn't use to be an issue because the client was only used for
private endpoint configuration, which did not use account key for auth,
so validation was more predictable.

Author: flavianmissi