Backup hosts don't get used if $NDSSERVER doesn't have the data

Created by: e-q

Installing gwpy on a 40m machine here. Our $NDSSERVER environment variable is set to point at our local frame builder, which doesn't mirror any aLIGO data.

gwpy.io.nds.host_resolution_order only tries to use the server in $NDSSERVER, so if that server doesn't have the channel, fetch just fails, rather than trying the other known servers.

Of course, specifying the host kwarg works, but I'm lazy :P I'm about to propose a PR that is one way to fix this.