[chassis/dvs] Add support for virtual chassis to DVS testbed (sonic-n…

…et#1345)

* Add a test_virtual_chassis.py to verify the virtual chassis
* Support virtual chassis topology in conftest.py to run the test_virtual_chassis.py. Notable changes in conftest.py are:
    - Unlike create_vnet, virtual_chassis does not use a sw container.
    - New DockerVirtualChassisTopology class to setup the connections when the virtual chassis containers restart.
* Integrate virtual chassis into conftest.py

tests/test_virtual_chassis.py

@@ -0,0 +1,26 @@
+import pytest
+
+class TestVirtualChassis(object):
+    def test_connectivity(self, vct):
+        if vct is None:
+            return
+        dvss = vct.dvss
+        nbrs = vct.get_topo_neigh()
+        for name in dvss.keys():
+            dv = dvss[name]
+            #ping all vs's inband address
+            for ctn in vct.inbands.keys():
+                ip = vct.inbands[ctn]["inband_address"]
+                ip = ip.split("/")[0]
+                print("%s: ping inband address %s" % (name, ip))
+                _, out = dv.runcmd(['sh', "-c", "ping -c 5 -W 0 -q %s" % ip])
+                print(out)
+                assert '5 received' in out
+            if name not in nbrs.keys():
+                continue
+            for item in nbrs[name]:
+                ip = str(item[1])
+                print("%s: ping neighbor address %s" % (name, ip))
+                _, out = dv.runcmd(['sh', "-c", "ping -c 5 -W 0 -q %s" % ip])
+                print(out)
+                assert '5 received' in out