longhorn: [BUG] Some taint tolerations can not be set, but Longhorn should run on all nodes

Describe the bug It is not possible to set all possible taint tolerations. Note that tools like RKE set taints, e.g. node-role.kubernetes.io/etcd=true:NoExecute or node-role.kubernetes.io/controlplane=true:NoSchedule. The description in the Longhorn UI says

Because kubernetes.io is used as the key of all Kubernetes default tolerations, it should not be used in the toleration settings.

This suggests that such tolerations can be set - but in fact, it is not allowed at all. And my understanding from other bug reports is that Longhorn should be deployed on all nodes (see e.g. https://github.com/longhorn/longhorn/issues/1633#issuecomment-702967887) because a pod using a Longhorn volume could be deployed anywhere (and have special tolerations). Imho this is contradictory.

To Reproduce Steps to reproduce the behavior:

1. Go to the Longhorn UI
2. Set a toleration like node-role.kubernetes.io/controlplane=true:NoSchedule, try to save changes
3. Validation fails with

fail to set settings with invalid taint-toleration: value node-role.kubernetes.io/controlplane=true:NoSchedule of settings taint-toleration is invalid: the value of taint-toleration is invalid: the key of Longhorn toleration setting cannot contain “kubernetes.io” since this substring is considered as the key of Kubernetes default tolerations

Expected behavior

• allow taint toleration for all taints that can exist
• also allow wildcard tolerations - see https://kubernetes.io/docs/concepts/scheduling-eviction/taint-and-toleration/#concepts (“There are two special cases” box)
• (potentially) improve description in Longhorn UI

Log

Environment:

• Longhorn version: 1.0.2
• Kubernetes version: 1.18.9
• Node OS type and version: Ubuntu 18.04

Additional context See https://github.com/longhorn/longhorn-manager/blob/55c8453c22b0f47da54bb6922e0ac97c37154b75/upgrade/v1alpha1/types/setting.go#L439 (validation rule and code for parsing)