support running commands with the same environment a user would
This comes out of #10600. While we were able to generate the necessary PATH info in the workunit.py task to solve that problem, it's going to continue to be a problem in future that what we run under teuthology has different environment settings than what we run interactively. (For instance, we pretty frequently stick an "lsof" command in the middle of tests which are failing due to open files.)
Testing has confirmed that "ssh bash -lc" does get you the proper environment set up, but handling the quoting so things are interpreted at the right level can be annoying and I'm not sure what the impact of invoking many of our other wrappers inside a subsidiary shell might be.