do not attempt to install resolv.conf on fs if DISABLE_EXPORTS_SETUP

In install/phases/boss/{ssh,sslcerts} we already use the setting of DISABLE_EXPORTS_SETUP to determine if emulab is managing the fs node (and thus can install files on it) or if the fs node is not managed by boss and we should not try and install files on it (e.g. if the fs node is a netapp filer). Add a test for this to install/phases/boss/named so that we don't try to install an /etc/resolv.conf on fs if DISABLE_EXPORTS_SETUP is set.