Added environment sanity check to parallel test runner.
Added environment sanity check to parallel test runner.

This patch adds a check to the parallel test runner that catches
setups where the user is not allowed to launch many process (e.g., the
default value for `ulimit -u` on centos7).

This can lead to test execution failures if many `mesos-tests`
instances are run in parallel on machines with many cores (by default
the parallel test runner would execute `nproc` shards which would fork
potentially multiple executable launching `nproc` libprocess worker
threads).

Review: https://reviews.apache.org/r/69275/

dforsyth/mesos,abudnik/mesos,gsantovena/mesos,kaysoky/mesos,Gilbert88/mesos,kaysoky/mesos,kaysoky/mesos,reneploetz/mesos,Gilbert88/mesos,dforsyth/mesos,jpeach/mesos,jpeach/mesos,gsantovena/mesos,kaysoky/mesos,gsantovena/mesos,gsantovena/mesos,dforsyth/mesos,chhsia0/mesos,jpeach/mesos,dforsyth/mesos,abudnik/mesos,chhsia0/mesos,chhsia0/mesos,chhsia0/mesos,gsantovena/mesos,reneploetz/mesos,dforsyth/mesos,kaysoky/mesos,abudnik/mesos,abudnik/mesos,Gilbert88/mesos,Gilbert88/mesos,reneploetz/mesos,Gilbert88/mesos,abudnik/mesos,jpeach/mesos,dforsyth/mesos,reneploetz/mesos,chhsia0/mesos,jpeach/mesos,jpeach/mesos,abudnik/mesos,Gilbert88/mesos,Gilbert88/mesos,chhsia0/mesos,reneploetz/mesos,dforsyth/mesos,jpeach/mesos,kaysoky/mesos,gsantovena/mesos,abudnik/mesos,kaysoky/mesos,reneploetz/mesos,gsantovena/mesos,reneploetz/mesos,chhsia0/mesos