S3 images can now be bootstrapped outside EC2.
However, this fix adds the requirement of a "region" setting.
At some point we could make "region" optional and fail when bootstrap-vz
is run outside EC2 and no "region" is set.
automatically from the first available partition.
There is no need to fiddle with sizes while keeping the offsets in mind any longer.
Introduced Bytes() class which makes it a lot easier to handle size units.
