Extend `min_allocatable_resources` flag to cover non-scalar resources.

Users will be able to specify non-scalar resources such as ports. This will help the framework to have more control over the shape of the offer.

Specifically, setting the flag to e.g. `cpus:0.01;mem:32;port:1` would make sure all offers will contain cpus, memory and ports. This will help alleviate the port starvation issue mentioned in MESOS-9324.