Revert PR#11831 (#12035) "Upgrade docker-sonic-mgmt base image from Ubuntu18.04 to 20.04"

Author: wsycqyz

dockers/docker-sonic-mgmt/Dockerfile.j2

@@ -1,5 +1,5 @@
 {% set prefix = DEFAULT_CONTAINER_REGISTRY %}
-FROM {{ prefix }}ubuntu:20.04
+FROM {{ prefix }}ubuntu:18.04
 
 ENV DEBIAN_FRONTEND=noninteractive
 
@@ -21,6 +21,8 @@ RUN apt-get update && apt-get install -y build-essential \
                                          psmisc \
                                          python \
                                          python-dev \
+                                         python-scapy \
+                                         python-pip \
                                          python3-pip \
                                          python3-venv \
                                          rsyslog \
@@ -29,16 +31,10 @@ RUN apt-get update && apt-get install -y build-essential \
                                          sudo \
                                          tcpdump \
                                          telnet \
-                                         vim \
-                                         python-is-python2 \
-                                         software-properties-common
-
-RUN add-apt-repository -y universe
-RUN curl https://bootstrap.pypa.io/pip/2.7/get-pip.py --output get-pip.py \
-    && python2 get-pip.py
+                                         vim
 
 RUN pip install setuptools==44.1.1
-RUN pip install cffi==1.12.0 \
+RUN pip install cffi==1.10.0 \
                 contextlib2==0.6.0.post1 \
                 cryptography==3.3.2 \
                 "future>=0.16.0" \
@@ -100,7 +96,7 @@ RUN pip install cffi==1.12.0 \
     && rm -f 1.0.0.tar.gz  \
     && pip install nnpy    \
     && pip install dpkt    \
-    && pip install scapy==2.4.5 --upgrade --ignore-installed
+    && pip install scapy==2.4.5 --upgrade
 
 # Install docker-ce-cli
 RUN apt-get update                  \
@@ -131,7 +127,7 @@ debs/{{ deb }}{{' '}}
 {%- endfor -%}
 debs/
 
-RUN dpkg --force-all -i \
+RUN dpkg -i \
 {% for deb in docker_sonic_mgmt_debs.split(' ') -%}
 debs/{{ deb }}{{' '}}
 {%- endfor %}
@@ -197,7 +193,8 @@ ENV PATH="$VIRTUAL_ENV/bin:$PATH"
 
 ENV LANG=C.UTF-8 LC_ALL=C.UTF-8 PYTHONIOENCODING=UTF-8
 
-RUN python3 -m pip install --upgrade --ignore-installed pip setuptools==58.4.0 wheel==0.33.6
+RUN python3 -m pip install --upgrade --ignore-installed pip setuptools==58.4.0
+
 RUN python3 -m pip install setuptools-rust \
                             aiohttp \
                             defusedxml \
@@ -240,6 +237,7 @@ RUN python3 -m pip install setuptools-rust \
                             tabulate \
                             textfsm==1.1.2 \
                             virtualenv \
+                            wheel==0.33.6 \
                             pysubnettree \
                             nnpy \
                             dpkt \