fix python3 warnings conftest and test_virtual_chassis

in conftest, use the utf-8 decode when reading the output
in test_virtual_chassis, iter no longer available in python3

Author: saravanansv

tests/conftest.py

@@ -1151,7 +1151,7 @@ def ctn_runcmd(self, ctn, cmd):
         res = ctn.exec_run(cmd)
         try:
             exitcode = res.exit_code
-            out = res.output
+            out = res.output.decode('utf-8')
         except AttributeError:
             exitcode = 0
             out = res

tests/test_virtual_chassis.py

@@ -6,7 +6,7 @@ def test_connectivity(self, vct):
             return
         dvss = vct.dvss
         nbrs = vct.get_topo_neigh()
-        for name in dvss.iterkeys():
+        for name in dvss.keys():
             dv = dvss[name]
             #ping all vs's inband address
             for ctn in vct.inbands.keys():