allow for direct use of singularity containers on CVMFS on compute site

Hi Karan,

I've been experimenting with the current singularity implementation in 4.8.4 and hit the following problem. I want to use Brian's images from /cvmfs/singularity.opensciencegrid.org/pycbc/pycbc-el7:latest as they are pre-staged to all of our nodes, but I get the error

2018.10.12 09:06:43.922 EDT: [FATAL ERROR]
[1]: Unable to instantiate Transformation Catalog at edu.isi.pegasus.planner.catalog.transformation.TransformationFactory.loadInstance(TransformationFactory.java:172)
[2]: edu.isi.pegasus.planner.catalog.transformation.impl.Text caught edu.isi.pegasus.planner.parser.ScannerException line 28: at edu.isi.pegasus.planner.parser.TransformationCatalogTextParser.parse(TransformationCatalogTextParser.java:236)
[3]: Invalid suffix .opensciencegrid.org/pycbc/pycbc-el7:latest determined singularity image url url -> file:///cvmfs/singularity.opensciencegrid.org/pycbc/pycbc-el7:latest , protocol -> file , host -> , path -> /cvmfs/singularity.opensciencegrid.org/pycbc/pycbc-el7:latest , url-prefix -> file:// at edu.isi.pegasus.planner.catalog.transformation.classes.Container.computeLFN(Container.java:180)

It looks like Pegasus doesn't like the file URL in my transformation catalog:

tr pegasus::findrange:4.0 {

 site condorpool {

   pfn "gsiftp://sugwg-condor.phy.syr.edu/usr/bin/pegasus-keg"
   arch "x86_64"
   os "linux"

   # INSTALLED means pfn refers to path in the container.
   # STAGEABLE means the executable can be staged into the container
   type "STAGEABLE"

   #optional attribute to specify the container to use
   container "centos-pycbc"
 }
}

cont centos-pycbc{
    # can be either docker or singularity
    type "singularity"

    # URL to image in a docker|singularity hub OR
    # URL to an existing docker image exported as a tar file or singularity image
    image "file:///cvmfs/singularity.opensciencegrid.org/pycbc/pycbc-el7:latest"

    # optional site attribute to tell pegasus which site tar file
    # exists. useful for handling file URL's correctly
    image_site "pycbc"
}

Cheers,
Duncan.