[201911] Fix easy_install error when installing pip (sonic-net#7272)

see below error:

+ sudo https_proxy= LANG=C chroot ./fsroot easy_install pip==20.3.3
Searching for pip==20.3.3
Reading https://pypi.python.org/simple/pip/
Couldn't find index page for 'pip' (maybe misspelled?)
Scanning index of all packages (this may take a while)
Reading https://pypi.python.org/simple/
No local packages or working download links found for pip==20.3.3
error: Could not find suitable distribution for Requirement.parse('pip==20.3.3')

How I fix:

Install python-pip via apt-get
Pin the version to 20.3.3
Master has same changes.

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

build_debian.sh

@@ -315,6 +315,7 @@ sudo LANG=C DEBIAN_FRONTEND=noninteractive chroot $FILESYSTEM_ROOT apt-get -y in
     ndisc6                  \
     makedumpfile            \
     conntrack               \
+    python-pip              \
     jq
 
 
@@ -413,7 +414,7 @@ if [[ $CONFIGURED_ARCH == amd64 ]]; then
 fi
 
 ## docker-py is needed by Ansible docker module
-sudo https_proxy=$https_proxy LANG=C chroot $FILESYSTEM_ROOT easy_install 'pip==20.3.3'
+sudo https_proxy=$https_proxy LANG=C chroot $FILESYSTEM_ROOT pip install --upgrade 'pip==20.3.3'
 sudo https_proxy=$https_proxy LANG=C chroot $FILESYSTEM_ROOT pip install 'docker-py==1.6.0'
 ## Note: keep pip installed for maintainance purpose