Add Azure pipeline to build legacy sonic-mgmt-docker (sonic-net#17073)

Why I did it
This is part of Python3 migration project.
This pipeline will build sonic-mgmt-docker with both Python2 and Python3.

The main difference between legacy sonic-mgmt-docker and now is:
make LEGACY_SONIC_MGMT_DOCKER=y target/docker-sonic-mgmt.gz
docker tag docker-sonic-mgmt $REGISTRY_SERVER/docker-sonic-mgmt:legacy

Work item tracking
Microsoft ADO (number only): 25254349

How I did it
Add pipeline file.

.azure-pipelines/docker-sonic-mgmt-legacy.yml

@@ -0,0 +1,63 @@
+# Starter pipeline
+# Start with a minimal pipeline that you can customize to build and deploy your code.
+# Add steps that build, run tests, deploy, and more:
+# https://aka.ms/yaml
+# Build and push sonic-mgmt image
+
+schedules:
+- cron: "0 8 * * *"
+ branches:
+ include:
+ - master
+ always: true
+
+trigger: none
+pr:
+ branches:
+ include:
+ - master
+ paths:
+ include:
+ - dockers/docker-sonic-mgmt
+
+parameters:
+- name: registry_url
+ type: string
+ default: sonicdev-microsoft.azurecr.io
+- name: registry_conn
+ type: string
+ default: sonicdev
+
+stages:
+- stage: Build
+ jobs:
+ - job: Build
+ pool: sonicbld
+ timeoutInMinutes: 360
+ steps:
+ - template: cleanup.yml
+ - checkout: self
+ clean: true
+ - bash: |
+ set -xe
+ git submodule update --init --recursive -- src/sonic-platform-daemons src/sonic-genl-packet src/sonic-sairedis src/ptf src/sonic-device-data
+
+ make SONIC_BUILD_JOBS=$(nproc) DEFAULT_CONTAINER_REGISTRY=publicmirror.azurecr.io ENABLE_DOCKER_BASE_PULL=y configure PLATFORM=generic
+ make SONIC_BUILD_JOBS=$(nproc) DEFAULT_CONTAINER_REGISTRY=publicmirror.azurecr.io ENABLE_DOCKER_BASE_PULL=y LEGACY_SONIC_MGMT_DOCKER=y target/docker-sonic-mgmt.gz
+ cp target -r $(Build.ArtifactStagingDirectory)/target
+ docker load -i target/docker-sonic-mgmt.gz
+ docker tag docker-sonic-mgmt $REGISTRY_SERVER/docker-sonic-mgmt:legacy
+ env:
+ REGISTRY_SERVER: ${{ parameters.registry_url }}
+ displayName: Build docker-sonic-mgmt.gz
+ - task: Docker@2
+ displayName: Upload image
+ inputs:
+ containerRegistry: ${{ parameters.registry_conn }}
+ repository: docker-sonic-mgmt
+ command: push
+ tags: latest
+ - publish: $(Build.ArtifactStagingDirectory)
+ artifact: 'docker-sonic-mgmt'
+ displayName: "Archive docker image sonic-mgmt"
+