Add new docker-sonic-mgmt makefile flag: LEGACY_SONIC_MGMT_DOCKER (#17070)

Why I did it
This is part of Python3 migration project. This PR will add a new makefile flag: LEGACY_SONIC_MGMT_DOCKER
Now by default: LEGACY_SONIC_MGMT_DOCKER = y will build sonic-mgmt-docker with Python2 and Python3
If LEGACY_SONIC_MGMT_DOCKER = n will will sonic-mgmt-docker with Python3 only

Work item tracking
Microsoft ADO (number only): 25254349

How I did it
Add makefile flag: LEGACY_SONIC_MGMT_DOCKER

How to verify it
By default will build sonic-mgmt-docker with Python2 and Python3. No change compared to before.
Set LEGACY_SONIC_MGMT_DOCKER=n will build sonic-mgmt-docker with Python3 only

Makefile.work

@@ -147,6 +147,10 @@ ifeq ($(ENABLE_DOCKER_BASE_PULL),)
 	override ENABLE_DOCKER_BASE_PULL = n
 endif
 
+ifeq ($(LEGACY_SONIC_MGMT_DOCKER),)
+	override LEGACY_SONIC_MGMT_DOCKER = y
+endif
+
 ifneq ($(GZ_COMPRESS_PROGRAM), pigz)
 override GZ_COMPRESS_PROGRAM = gzip
 endif
@@ -572,6 +576,7 @@ SONIC_BUILD_INSTRUCTION :=  $(MAKE) \
                            ONIE_IMAGE_PART_SIZE=$(ONIE_IMAGE_PART_SIZE) \
                            SONIC_OS_VERSION=$(SONIC_OS_VERSION) \
                            PIP_HTTP_TIMEOUT=$(PIP_HTTP_TIMEOUT) \
+                           LEGACY_SONIC_MGMT_DOCKER=$(LEGACY_SONIC_MGMT_DOCKER) \
                            $(SONIC_OVERRIDE_BUILD_VARS)
 
 .PHONY: sonic-slave-build sonic-slave-bash init reset

dockers/docker-sonic-mgmt/Dockerfile.j2

@@ -1,6 +1,8 @@
 {% set prefix = DEFAULT_CONTAINER_REGISTRY %}
 FROM {{ prefix }}ubuntu:20.04
 
+{% set legacy = LEGACY_SONIC_MGMT_DOCKER %}
+
 ENV DEBIAN_FRONTEND=noninteractive
 
 RUN apt-get update && apt-get install -y apt-transport-https \
@@ -117,6 +119,7 @@ RUN python3 -m pip install aiohttp \
     && rm -f 1.2.tar.gz  \
     && python3 -m pip install nnpy
 
+{% if legacy == 'y' or legacy == '1' %}
 RUN curl -fsSL https://bootstrap.pypa.io/pip/2.7/get-pip.py --output get-pip.py \
     && python2 get-pip.py \
     && rm -f get-pip.py
@@ -185,6 +188,7 @@ RUN python2 -m pip install allure-pytest==2.8.22 \
                 textfsm==1.1.3 \
                 thrift==0.11.0 \
                 virtualenv
+{% endif %}
 
 # Install docker-ce-cli
 RUN curl -fsSL https://download.docker.com/linux/ubuntu/gpg | apt-key add -                                        \

slave.mk

@@ -443,6 +443,7 @@ $(info "BUILD_MULTIASIC_KVM"             : "$(BUILD_MULTIASIC_KVM)")
 endif
 $(info "CROSS_BUILD_ENVIRON"             : "$(CROSS_BUILD_ENVIRON)")
 $(info "GZ_COMPRESS_PROGRAM"             : "$(GZ_COMPRESS_PROGRAM)")
+$(info "LEGACY_SONIC_MGMT_DOCKER"        : "$(LEGACY_SONIC_MGMT_DOCKER)")
 $(info )
 else
 $(info SONiC Build System for $(CONFIGURED_PLATFORM):$(CONFIGURED_ARCH))