Parallel test runner now the default for autotools / make check

[Benjamin Mahler <bm...@apache.org>]

During the MesosCon hackathon Benjamin Bannier and myself worked on getting
the parallel test runner usable as the default for the autotools build.

Now, when running 'make check', the tests will run much much faster!

What we did:

-added detection of 'ulimit -u' and exit if too low
-fixed some flaky tests
-changed the default 'configure' behavior to require opt-out of the
parallel test runner
-fixed reviewbot to oot-

What's still left to do:

-fix any other flaky tests that crop up
-update CMake build to use the parallel test runner
-improve the output and subprocess management behavior of the runner

Give it a shot (might need to reconfigure before running make check, or you
can run it manually through CMake build) and raise any issues you see.

bmahler & bbannier