--s3-use-glacier and --no-encryption cause slow backups

If S3 Glacier support and no GPG encryption are selected, the remote manifest file is incorrectly marked as GLACIER. This causes all operations on the manifest to be very slow (order hours).

This code in backends/_boto_single.py seems to be responsible:

        elif globals.s3_use_glacier and u"manifest.gpg" not in remote_filename:
            storage_class = u'GLACIER'

Suggested fix is to match more precisely on the remote_filename:

        elif globals.s3_use_glacier and not (remote_filename.endswith(u"manifest.gpg") or remote_filename.endswith(u"manifest"):
            storage_class = u'GLACIER'

Maybe even move this logic to a ismanifest() function somewhere?

Observed in Duplicity 0.8.05 on Debian Buster