Delay a random jitter period prior to request a Router ID for REED. (#554)

* Delay a random jitter period prior to request a Router ID for REED.

* Update Cert_5_1_09_REEDAttachConnectivity.py to pass Travis check.

* Add CLI to set/get the ROUTER_SELECTION_JITTER and update Certification scripts.

  - add 'posix-ncp' to allow failure list in .travis.yml.

tests/scripts/thread-cert/node.py

@@ -260,6 +260,11 @@ class Node:
         self.send_command(cmd)
         self.pexpect.expect('Done')
 
+    def set_router_downgrade_threshold(self, threshold):
+        cmd = 'routerdowngradethreshold %d' % threshold
+        self.send_command(cmd)
+        self.pexpect.expect('Done')
+
     def release_router_id(self, router_id):
         cmd = 'releaserouterid %d' % router_id
         self.send_command(cmd)
@@ -402,5 +407,10 @@ class Node:
 
         return result
 
+    def set_router_selection_jitter(self, jitter):
+        cmd = 'routerselectionjitter %d' % jitter
+        self.send_command(cmd)
+        self.pexpect.expect('Done')
+
 if __name__ == '__main__':
     unittest.main()