Add 3 pipeline files:
- pipeline for build docker image sonic-slave-[buster|jessie|stretch] for amd64/armhf/arm64, and push to ACR(sonicdev-microsoft.com)
- pipeline for build docker image sonic-mgmt, and push to ACR
- pipeline for cleaning dpkg cache which are created more than 30 days.
Co-authored-by: lguohan <lguohan@gmail.com>
