vty: Close the vty sockets at the end of each test

In the OpenBSC VTY tests it seems no one is calling
the command with close=True. So make sure the socket
is released. Modify _close_socket to be idempotent.

Change-Id: I717537772e89113ef352d32ffba1b7a840f2c6e6

osmopy/obscvty.py

@@ -43,8 +43,9 @@ class VTYInteract(object):
         self.last_node = ''
 
     def _close_socket(self):
-        self.socket.close()
-        self.socket = None
+        if self.socket:
+            self.socket.close()
+            self.socket = None
 
     def _is_end(self, text, ends):
         """

osmopy/osmotestvty.py

@@ -48,6 +48,7 @@ class TestVTY(unittest.TestCase):
         self.vty = obscvty.VTYInteract(appstring, "127.0.0.1", appport)
 
     def tearDown(self):
+        self.vty._close_socket()
         self.vty = None
         osmoutil.end_proc(self.proc)