[201911] Fix PTF Docker Build Error (#6583)

We are hitting the issue as described pypa/pip#9520.
Fix to use get_pip.py from 2.7 repo.

Signed-off-by: Abhishek Dosi <abdosi@microsoft.com>

dockers/docker-ptf/Dockerfile.j2

@@ -77,7 +77,7 @@ RUN rm -rf /debs \
     && apt-get -y autoclean \
     && apt-get -y autoremove \
     && rm -rf /var/lib/apt/lists/* \
-    && wget --https-only https://bootstrap.pypa.io/get-pip.py \
+    && wget --https-only https://bootstrap.pypa.io/2.7/get-pip.py \
     && python get-pip.py \
     && rm -f get-pip.py \
     && pip install setuptools \