Extended information about node processes

It'll be better if plugin will return extended information about node processes to improve savanna-side validation and to add some additional validation at UI side.

Here is a list of potential constraints for node processes:

* min_instances (1 / None) - means that each cluster should have at least N instances of the specific node process;

* max_instances (2 / None) - means that each cluster should have maximum N instances of the specific node process;

* is_scalable (True / False) - defines could the specific node process be scaled or not;

* anti_affinity_applicable (True / False) - should we display specific process in the list of AA processes? (looks like it's not needed if we'll have min/max instance).

TBD: add more details about Plugin SPI and REST API