Multi-tenancy model

[binbin-li]

Ratify is adding multi-tenancy support to provide better user experience to customers. There is an on-going discussion on this topic, and a PR out to address it.

In general, there are 2 options we can adopt 2 Ratify deployment strategies for the multi-tenancy support. One is to deploy Ratify service in each namespace, and the other is to deploy a centralized Ratify in the cluster.

A brief comparison of 2 strategies is as below:

	Deployment per Namespace	Deployment per Cluster
Core workflow refactoring	easy	hard
Helm templates refactoring	hard	easy
ED request struct	no change	add namespace
ED request number	1 request per Ratify deployment	1 request
Resouce Quota	easy	hard
Log isolation	easy	hard
Cache isolation	easy	hard
Metrics isolation	easy	hard
Resouce efficiency	low	high
User Experience	hard	easy
Ratify configuration control	fine-grained control	limited

More detailed comparison can be checked in the above PR.