Always run iptables-save as root

When there is a failure and we run commands to collect
network information, the iptables-save command always
fails in the root namespace:

  Executing command 'iptables-save' on local host (timeout=None)...
  Command 'iptables-save' failed (exit_status=4):
  stderr:
  iptables-save v1.8.7 (nf_tables): Could not fetch rule set
    generation id: Permission denied (you must be root)

Change list_iptables() to always use sudo fixes the issue.

Trivialfix

Change-Id: I656569cb333e98f5f55b51b0cfec8e99922424b3
diff --git a/neutron_tempest_plugin/common/ip.py b/neutron_tempest_plugin/common/ip.py
index e2f6a4a..07bbe69 100644
--- a/neutron_tempest_plugin/common/ip.py
+++ b/neutron_tempest_plugin/common/ip.py
@@ -391,9 +391,9 @@
 
 
 def list_iptables(version=constants.IP_VERSION_4, namespace=None):
-    cmd = ''
+    cmd = 'sudo '
     if namespace:
-        cmd = 'sudo ip netns exec %s ' % namespace
+        cmd += 'ip netns exec %s ' % namespace
     cmd += ('iptables-save' if version == constants.IP_VERSION_4 else
             'ip6tables-save')
     return shell.execute(cmd).stdout