HEAD-1867 fix detection of image filename from manifest
HEAD-1867 fix detection of image filename from manifest

kaizengarden/sdc-headnode,kaizengarden/sdc-headnode,joyent/sdc-headnode,numericillustration/sdc-headnode,numericillustration/sdc-headnode,joyent/sdc-headnode,numericillustration/sdc-headnode,kaizengarden/sdc-headnode,kaizengarden/sdc-headnode,joyent/sdc-headnode,joyent/sdc-headnode