Make netns_wrapper to be compatible with python3

netns_wrapper is using map() function when running command and this
returns map object instead of list in python 3. This patch fixes it
to return list object always.

Change-Id: I22ddde78f2d8af042789e52ea82ec3e8099f7888

neutron_vpnaas/services/vpn/common/netns_wrapper.py

@@ -58,7 +58,7 @@ def setup_conf():
 def execute(cmd):
     if not cmd:
         return
-    cmd = map(str, cmd)
+    cmd = list(map(str, cmd))
     LOG.debug("Running command: %s", cmd)
     env = os.environ.copy()
     obj = utils.subprocess_popen(cmd, shell=False,