run tests_run in parallel

python pytest-xdist; we haven't been using this;
tests_run_all seem to fail with this (Swarming already)
add external cmake module to determin number for CPU cores
execute python unittests in parallel

pytest-xdist -n
Merge branch 'master' into parallel_unittests
Revert "add external cmake module to determin number for CPU cores"

This reverts commit f46362b6e31fd460d5149a812c4c36d2b11fac92.
Revert "execute python unittests in parallel"

This reverts commit a05f97b4013971e5cf0229b07b0ab018ed6bf3b6.
parallel tests: use harcoded 2 threads

not much improvement was observed otherwise.
Merge branch 'master' into parallel_unittests

Conflicts:
	CMakeLists.txt
fix mistake in merge
Merge branch 'master' into parallel_unittests
Merge pull request #958 from breznak/parallel_unittests

Parallel unittests