Aptomi core & model cleanup

• [x] #253 Do not allow to change a cluster via API
• [ ] #270 Introduce namespaces as entities (ACL rules should live inside a NS)
• [x] #282 Validate component instances during policy resolution
• [x] #278 Add a notion of deployment target, remove NS from cluster definition
• [ ] #205 Improve service discovery when contract is implemented via different services
• [x] #302 Agree on the new terminology and rename core entities
• [x] #303 Use label “target” instead of “cluster” to specify placement
• [ ] #307 Remove user from dependency
• [ ] #305 Make service sharing more explicit in policy language
• [ ] #306 kafka.kafka in discovery + input/output parameters for the contract
• [x] #318 Rework how we create/save revisions and desired state
• [x] #321 Better update logic for transferring data from desired state to actual state
• [ ] #335 Support clusters local to namespaces, not only in system namespace
• [x] #327 Figure out EdgesIn & EdgesOut in actual state