Ensure that python36-psutil is present in docker build environment

mockchain-parallel imports psutil.  However the version of python
to be used for mockchain-parallel is set by mock, and could be
2.7 or 3.6.  The mock package itself doesn't require psutil so
we must import it ourself, and make sure both are present.

Change-Id: I7c5d9b52eed49cdce02224603b9bfd17e4426558
Signed-off-by: Scott Little <scott.little@windriver.com>

Dockerfile

@@ -75,6 +75,8 @@ RUN groupadd -g 751 cgts && \
         python-pep8 \
         python-pip \
         python-psutil \
+        python2-psutil \
+        python36-psutil \
         python-sphinx \
         python-subunit \
         python-pip \