Prevent add TiFlash Nodes without placement rules

[disksing]

Feature Request

Describe your feature request related problem:

TiFlash nodes cannot become follower or learner. We use placement rules to prevent moving followers or leaders to TiFlash nodes. If user forget to set up placement rules before adding TiFlash nodes, it will cause TiFlash nodes crash.

Describe the feature you'd like:

• When placement rules not enabled, don't allow add tiflash node.
• When the cluster has tiFlash nodes, don't allow disable placement rules.

Describe alternatives you've considered:

Teachability, Documentation, Adoption, Migration Strategy: