Add more validation for Plugin entity

- Add jsonchema for plugin handlers
- Add additional validation of cluster-plugin compatibility
  Validates on 3 parameters:
  - cluster operating system
  - cluster mode
  - release version of cluster
- Plugin can provide UI native conditions to hide/show plugins

related to blueprint cinder-neutron-plugins-in-fuel

Change-Id: Ic6dc24a133478bf89dfdc8f07f3fb2cfe8fdcb52
Merge "Add more validation for Plugin entity"