chg: [threat-actor] fix #561 by using new meta to classify as a campaign only.

Based on https://github.com/MISP/misp-galaxy/issues/469

There is an old and persistence issue in attribution world and basically no-one really agrees on this. So we decided to start a specific metadata `threat-actor-classification` on the threat-actor to define the various types per cluster entry:

- _operation_:
  - _A military operation is the coordinated military actions of a state, or a non-state actor, in response to a developing situation. These actions are designed as a military plan to resolve the situation in the state or actor's favor. Operations may be of a combat or non-combat nature and may be referred to by a code name for the purpose of national security. Military operations are often known for their more generally accepted common usage names than their actual operational objectives._ from Wikipedia
  - **In the context of MISP threat-actor name, it's a single specific operation.**
- _campaign_:
  - _The term military campaign applies to large scale, long duration, significant military strategy plans incorporating a series of inter-related military operations or battles forming a distinct part of a larger conflict often called a war. The term derives from the plain of Campania, a place of annual wartime operations by the armies of the Roman Republic._ from Wikipedia
  - **In the context of MISP threat-actor-name, it's long-term activity which might be composed of one or more operations.**
- threat-actor
  - **In the context of MISP threat-actor-name, it's an agreed name by a set of organisations.**
- activity group
  - **In the context of MISP threat-actor-name, it's a group defined by its set of common techniques or activities.**
- unknown
  - **In the context of MISP threat-actor-name, it's still not clear if it's an operation, campaign, threat-actor or activity group**

The meta field is an array to allow specific cluster of threat-actor to show the current disagreement between different organisations about the type (threat actor, activity group, campaign and operation).

clusters/threat-actor.json

@@ -6810,6 +6810,9 @@
         ],
         "synonyms": [
           "Roaming Mantis Group"
+        ],
+        "threat-actor-classification": [
+          "campaign"
         ]
       },
       "uuid": "b27beb94-ce25-11e8-8e11-2f1a59bd0e91",
@@ -8356,5 +8359,5 @@
       "value": "GALLIUM"
     }
   ],
-  "version": 167
+  "version": 168
 }