pegasus-transfer should support docker->singularity pulls

the passed type attribute on destination url will be
- cont.docker OR
  cont.singularity

here is the same format
 { "type": "transfer",
   "linkage": "input",
   "lfn": "centos-pegasus.simg",
   "id": 1,
   "src_urls": [
     { "site_label": "local", "url": "file:///Volumes/Work/lfs1/work/pegasus-features/PM-1159/work/vahi/pegasus/blackdiamond/run0018/00/00/centos-pegasus.simg", "checkpoint": "false" }
   ],
   "dest_urls": [
     { "site_label": "local", "url": "symlink://$PWD/centos-pegasus.simg" "type": "cont.singularity" }
   ] }