Do not store a *check.C in openshiftCluster

The *check.C object can not be reused across tests, so storing it in
openshiftCluster is incorrect (and leads to weird behavior like
assertion failures being silently ignored).  So far this hasn't really
been an issue because we have been using the *check.C only in SetUpSuite
and TearDownSuite, and the changes to this have turned out to be
unnecessary after all, but this is still the right thing to do.

This is more or less
> s/c\./cluster\./g; s/cluster\.c/c/g
(paying more attention to the syntax) and corresponding modifications
to the method declarations.

Does not change behavior, apart from using the correct *check.C in
CopySuite.TearDownSuite.

integration/copy_test.go

@@ -75,7 +75,7 @@ func (s *CopySuite) TearDownSuite(c *check.C) {
 		s.registry.Close()
 	}
 	if s.cluster != nil {
-		s.cluster.tearDown()
+		s.cluster.tearDown(c)
 	}
 }