[PM-1735] Add support for custom Singularity library locations

[mayani]

From user:

Yes, we have our own Singularity repository, but this will be used by all users. However, to use this repository so far I’ve been adding a new Singularity “remote” library and then setting it as the active one with “remote use”. My concern was whether each user of the system would have to do this so that when a Pegasus workflow is executed as that user, the right remote repository is used to pull the necessary images.

However, I heard back on our GitHub ticket where they suggested using “global” remotes, and this seems to work (https://github.com/hpcng/singularity/issues/5882).

Reporter: @rynge Watchers: @rynge

[vahi]

@rynge are we planning to do this?

[rynge]

Won't fix for now.