Add koji's python reqs to centos6 docker image

This will allow, but not require, running tests using only RPM-packaged
requirements.

Signed-off-by: Zack Cerza <zack@redhat.com>

devtools/containers/centos/Dockerfile

@@ -8,7 +8,9 @@ RUN \
     libffi-devel \
     openssl-devel \
     pyOpenSSL \
+    python-cheetah \
     python-devel \
+    python-requests \
     redhat-rpm-config \
     rpm-build \
     rpm-python \
@@ -16,6 +18,7 @@ RUN \
   yum install -y epel-release && \
   yum install -y \
     python-flake8 \
+    python-multilib \
     python-pip \
     python-psycopg2 \
     python-qpid-proton \