Add 'discover' mode to fl-run-bench

Discover mode will let fl-run-bench be invoked without specifying the
particular test case. This implementation attempts to behave like
unittest's 'discover' argument, and there is even a similar --failfast
option.

I also shuffled around / refactored some code and made some slight
tweaks to the help / usage text.
Don't pass 'discover' argument in distributed mode

Also, return failure if the distmgr object doesn't get created, rather
than going into the "finally:" and doing distmgr.abort()
better reproduce previous behaviour
Merge pull request #116 from sjbrown/discover-bench

Add 'discover' mode to fl-run-bench