Disabling it allows clang-3.4 to run C/C++ on docker

Notable Changes:
  -Convert all str types to byte types at cython layer (ascii encoding)
  -Use six for packages that have different names in Python2/Python3
  -By default, unit tests are compiled/run in Python2.7 and Python3.4
  -Ensure MACOSX_BUILD_TARGET is at least 10.7

This is for the same reasons as ParseJson, that is so that we can limit
the scope of the proto namespace differences between internal and
external.

Source packages were not getting the necessary C files due to
`make_grpcio_tools.py` not being invoked before `sdist` through
`setup.py`.

Before, namespace packages would break in auditwheel due to a
superfluous check (fixed in the commit referenced in the Dockerfiles).
Now, the auditwheel used in the manylinux1-support Docker images can
handle namespace packages, and we may thus use them. This should
alleviate future user pain w.r.t. installation of grpcio-tools causing
loss of grpcio files and vice versa (e.g. requiring a reinstall of
grpcio following installation of grpcio-tools).

If clients send GOAWAY followed by HEADER

This reverts commit f1cde580.

This reverts commit d88e15ce.