Fix tests that require sasl on docker

CentOS only supports glibc 2.12 but the sasl import was failing with:
ImportError: /lib64/libc.so.6: version `GLIBC_2.14' not found (required by /opt/rh/python27/root/usr/lib64/python2.7/site-packages/_saslwrapper.so)

To do this, the pip install command had to link to our Mozilla repo. This is better anyway since PyPI can go down and also poses some security concerns.

This patch also adds `&& yum clean` which saves a surprising amount of disk space in the image.

Dockerfile

@@ -1,7 +1,7 @@
 FROM mozillamarketplace/centos-mysql-mkt:0.2
 
 # Fix multilib issues when installing openssl-devel.
-RUN yum install -y --enablerepo=centosplus libselinux-devel
+RUN yum install -y --enablerepo=centosplus libselinux-devel && yum clean all
 
 ADD docker-mysql.repo /etc/yum.repos.d/mysql.repo
 
@@ -20,12 +20,16 @@ RUN yum update -y \
 
 # The version in the above image is ancient, and does not support the
 # --no-binary flag used in our requirements files.
-RUN pip install -U pip
+# We also need to install wheels.
+RUN pip install -U pip wheel
 
 COPY requirements /pip/requirements/
 RUN cd /pip && \
     pip install --build ./build --cache-dir ./cache \
-	--no-deps -r requirements/docker.txt && \
+        --find-links https://pyrepo.addons.mozilla.org/wheelhouse/ \
+        --find-links https://pyrepo.addons.mozilla.org/ \
+        --no-index --no-deps \
+        -r requirements/docker.txt && \
     rm -r build cache
 
 RUN mkdir /code