Nomad plan why-not

### Proposal

Implement a command (separate or as part of the job plan) that explains why a particular group cannot be placed on a particular node (something distantly similar to `aptitude why`). Possible syntax:

nomad job plan -why-not-group group1 -why-not-node node1 job.nomad

Output:

Allocation group1 cannot be placed on node node1 because it exhaust dimension "memory"

Possible additional option is something like `-why-not-count 3` to check why 3 allocations of a group cannot be placed on a particular node.

### Use-cases

Periodically I stumble across situations when a `nomad plan` shows that some allocation cannot be placed and it's not immediately clear why. Nomad message is fairly cryptic in even in verbose mode (and especially with a lot of nodes):

Task Group "nginx" (failed to place 1 allocation):
* Class "c1": 1 nodes excluded by filter
* Constraint "computed class ineligible": 1 nodes excluded by filter
* Resources exhausted on 2 nodes
* Class "c2" exhausted on 2 nodes
* Dimension "network: reserved port collision http=8272" exhausted on 1 nodes
* Dimension "memory" exhausted on 1 nodes

It will be time saving to have the ability to check why task group cannot be placed to specific node.

### Attempted Solutions

Currently it requires a lot of configs checks to get understanding why a job cannot be scheduled on a particular node. It is doable but can be quite time consuming.