Multicluster passwd-auth

[ma-efremoff]

UI should be able to use passwd-auth to login to each cluster from clusters-config.json.

TODO:

• [x] All yt-auth related controllers should have req.params.ytAuthCluster
• [x] YTCoreConfig.ytAuthCluster should replaced with YTCoreConfig.allowPasswordAuth
• [x] UI should be able to use cluster-specific settings
• [x] ClustersMenu should be available without any authentication
• [x] Add migration description for ytAuthCluster -> allowPasswordAuth
• [x] Fix description of ytInterfaceSecret
• [x] ytsaurus/yt-k8s-operator#162

About migration ytAuthCluster -> allowPasswordAuth

Additionaly we need some backward compatibility and some tips for users who do update from previous versions. Or the app should fall down with detailed description of what is wrong and how to fix it. Or the description should be sent to browser instead of any other response.