Move driver specific change status field to driver

The 'status' field is a driver specific field and should therefore be
defined and initialized in the driver specific subclass of a change and
not in the 'Change' base class.

Change-Id: I0a20041ae8b9f5e4c359fea86c21d1c5c5cbfaca

zuul/driver/gerrit/gerritmodel.py

@@ -31,6 +31,7 @@ EMPTY_GIT_REF = '0' * 40  # git sha of all zeros, used during creates/deletes
 class GerritChange(Change):
     def __init__(self, project):
         super(GerritChange, self).__init__(project)
+        self.status = None
         self.approvals = []
 
     def update(self, data, connection):

zuul/driver/github/githubmodel.py

@@ -47,11 +47,6 @@ class PullRequest(Change):
     def status(self):
         return ["{}:{}:{}".format(*c) for c in self.contexts]
 
-    @status.setter
-    def status(self, value):
-        # Dummy setter to ignore init to None in base class.
-        pass
-
     @property
     def successful_contexts(self) -> set:
         if not self.contexts:

zuul/driver/pagure/paguremodel.py

@@ -28,6 +28,7 @@ class PullRequest(Change):
         self.score = 0
         self.files = []
         self.tags = []
+        self.status = None
 
     def __repr__(self):
         r = ['<Change 0x%x' % id(self)]

zuul/model.py

@@ -3204,7 +3204,6 @@ class Change(Branch):
         self.is_merged = False
         self.failed_to_merge = False
         self.open = None
-        self.status = None
         self.owner = None
 
         # This may be the commit message, or it may be a cover message