Slurm plugin

[bcumming]

A large omnibus PR that adds support for SLURM.

• implement the slurm plugin
• CLI
  • many small improvements to the uenv start and uenv run commands
  • add the uenv image ls command
• testing:
  • setup scripts that create a test repo
  • install the bats bash testing framework for integration testing
  • integration tests for the slurm plugin only
• template and helper script for building the slurm plugin RPM

[simonpintarelli]

In bats, run sbatch --wait always returns status==0 (not clear why). Here is a workaround:
simonpintarelli@bcdf2a7

It greps for srun: error in the logfile instead.