Don't link TC to Issue Trackers if required parameters not present

By default these are api_url, api_username and api_password.
For GitHub they are base_url and api_password. Fixes #3

tcms/issuetracker/types.py

@@ -55,6 +55,12 @@ def add_testcase_to_issue(self, testcases, issue):
         """
         raise NotImplementedError()
 
+    def is_adding_testcase_to_issue_disabled(self):
+        """
+            When is linking a TC to a Bug disabled?
+        """
+        return not (self.tracker.api_url and self.tracker.api_username and self.tracker.api_password)
+
     def all_issues_link(self, ids):
         """
             Used in testruns.views.get() aka run/report.html to produce
@@ -249,6 +255,12 @@ def add_testcase_to_issue(self, testcases, issue):
         for case in testcases:
             github_integration.GitHubThread(self.rpc, self.tracker, case, issue).start()
 
+    def is_adding_testcase_to_issue_disabled(self):
+        """
+            Only disabled if we don't have an authentication token
+        """
+        return not (self.tracker.base_url and self.tracker.api_password)
+
     def report_issue_from_testcase(self, caserun):
         """
             GitHub only supports title and body parameters

tcms/testcases/models.py

@@ -333,7 +333,10 @@ def add_bug(self, bug_id, bug_system_id, summary=None, description=None,
             if bz_external_track:
                 bug_system = TestCaseBugSystem.objects.get(pk=bug_system_id)
                 it = IssueTrackerType.from_name(bug_system.tracker_type)(bug_system)
-                it.add_testcase_to_issue([self], bug)
+                if not it.is_adding_testcase_to_issue_disabled():
+                    it.add_testcase_to_issue([self], bug)
+                else:
+                    raise ValueError('Enable linking test cases by configuring API parameters for this Issue Tracker!')
         else:
             raise ValueError('Bug %s already exist.' % bug_id)