add ability to import existing firewalls

[hcwagner]

Currently the controller only supports a single firewall defined and owned by itself. The controller should be able to import an existing firewall when given the ID of the firewall to import.

Upon importing the firewall the controller should not manage the rules of the firewall, but instead only update the device list to include the nodes in the cluster.

[hcwagner]

Ability to import credentials via customer API token has been added. Another change is necessary to import a reference to an existing firewall ID.