use public API please

[nschlemm]

trying out collectfast with django-minio-storage, I stumbled over collectfast.etag.get_remote_etag() causing an
AttributeError: 'MinioStaticStorage' object has no attribute '_normalize_name'
which can easily be replaced by the public storages.utils.safe_join() which is called by storages.backends.s3boto3.S3Boto3Storage._normalize_name()

[antonagestam]

For reference, my ambitions has been for Collectfast to be usable for other storage engines as well, just never used anything else than S3 personally. Would be cool to introduce a concept of different strategies for different backends.

Moving away from private APIs is a great start :)