[Docker] Added support for python2 (#6753)

- Why I did it
Mellanox SDK APIs support python 2 at the moment.

- How I did it
Mellanox SDK APIs support python 2 at the moment.

- How to verify it
Add python 2 to Mellanox syncd only.

- Which release branch to backport (provide reason below if selected)
docker exec -t syncd /bin/bash -c "sx_api_dbg_generate_dump.py /home/sx_api_dbg_dump"
You can see that it will work and generate /home/sx_api_dbg_dump

Signed-off-by: allas <allas@nvidia.com>

platform/mellanox/docker-syncd-mlnx/Dockerfile.j2

@@ -11,7 +11,13 @@ ENV DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get update && \
     apt-get install -y \
-        libxml2
+        libxml2     \
+        python-pip  \
+        python-dev \
+        python-setuptools
+
+RUN pip2 install --upgrade pip
+RUN apt-get purge -y python-pip
 
 {% if docker_syncd_mlnx_debs.strip() -%}
 # Copy locally-built Debian package dependencies