Change PATH for "ip addr list" command so it could work with cloud-user

It's needed for some custom images like RHEL7 where /usr/sbin/ is not
enabled by default for users under test.

Change-Id: Ib75c468cc15f9912e110af4748734bf0b48bcf5d
Closes-bug: 1689279
diff --git a/neutron/tests/tempest/scenario/test_trunk.py b/neutron/tests/tempest/scenario/test_trunk.py
index 122068c..c857f3f 100644
--- a/neutron/tests/tempest/scenario/test_trunk.py
+++ b/neutron/tests/tempest/scenario/test_trunk.py
@@ -239,7 +239,8 @@
             # Configure VLAN interfaces on server
             command = CONFIGURE_VLAN_INTERFACE_COMMANDS % {'tag': vlan_tag}
             server['ssh_client'].exec_command(command)
-            out = server['ssh_client'].exec_command('ip addr list')
+            out = server['ssh_client'].exec_command(
+                'PATH=$PATH:/usr/sbin;ip addr list')
             LOG.debug("Interfaces on server %s: %s", server, out)
 
         # Ping from server1 to server2 via VLAN interface should fail because