Delete in reverse order

[ColonelPanics]

When a group is deployed it is deployed in alphabetical order of the nodes. When deleting it should do the reverse of deploy (to cope with any potential issues of dependent resources).

[WilliamMcCumstie]

Deleting the deployments links into the dependency management stuff. Possible a good idea to look at it first: #238

Similarly to the creation issue, the linking between "Deployments" needs to be handled correctly. For a first pass, only simple management is required. A more advanced issue will be opened with graph theory.

This time dependents instance method needs to be added to the Domain and Nodes. The Domain#dependents should return all the nodes. And Nodes#dependents should return an empty array.

Then when deleting a Node or Domain, then all the dependents need to be deleted first.

EDIT: Ignore this, see below

[WilliamMcCumstie]

As the issue initially states, the nodes should be deleted in reverse alphabetical order. But also, all the running deployments should be checked to see if they rely on the nodes being deleted. If they do, then an error should be raised