Trigger Mellanox CI with bot:[mellanox:]retest PR comments

[artemry-nv]

Currently Mellanox Open MPI CI is based on Azure Pipelines service and is triggered with /azp run PR comments (README).
For the convenience it's needed to investigate how to trigger it with the old PR comments bot:[mellanox:]retest.

Initial discussion: #7134

[artemry-nv]

Posted the related discussions about custom Azure Pipelines GitHub PR comment triggers on:

• Microsoft Developer Community forum: https://developercommunity.visualstudio.com/idea/905960/custom-pr-comment-triggers-for-azure-pipelines.html
• Stack Overflow: https://stackoverflow.com/questions/60061750/custom-github-pr-comment-triggers-for-azure-pipelines

[artemry-nv]

Microsoft representative has confirmed on Stack Overflow that custom PR comment triggers aren't supported and the special Azure Pipelines service connection type "Incoming Webhook" (to listen GitHub events such as new PR comment) isn't yet implemented. Submitted a feature request for this (link) - you can vote for this feature using the vote button.