Allow flexibility to define a target submodule and its branch
Closes rc/ohpc-packer#37 (closed)
This allows testing submodule (external for eg. github) feature branches in PRs for our builds by abstracting the feature Pull requests that need to be checked out, into gitlab CICD vars.
Build https://gitlab.rc.uab.edu/atlurie/ohpc_packer/-/jobs/41324#L44 succeded for this MR, It is checking PRs from github which is the purpose of this MR.
Edited by Eesaan Atluri