Use 'oc exec' when retrieving ovsdb command string

It was found that sometimes an unexpected output returned when
using 'oc rsh' on attempt to get ovsdb command string from db pod.
This was causing an AssertingError and a test failure.
Compared to 'oc rsh' the issue does not happen when using 'oc exec'
command.
Also running ps in the background using setsid helps to avoid
sporadic errors due to [1].

[1] https://github.com/kubevirt/kubevirt/issues/10240

Change-Id: I77530691613ca352ce5e5650b3a2450a7003f459

whitebox_neutron_tempest_plugin/tests/scenario/base.py

@@ -1243,11 +1243,13 @@ class BaseTempestTestCaseOvn(BaseTempestWhiteboxTestCase):
         command_items.append('--db="{}"'.format(all_db_addresses))
 
         # obtain the ovsdb-server command running on this pod
-        # `|| true` added to prevent the following issue:
+        # setsid added to prevent the following issue:
         # https://github.com/kubevirt/kubevirt/issues/10240
+        # `||true` ensures a safe exit.
         cmd = ' '.join((cls.OC,
-                        'rsh',
+                        'exec',
                         db_pod,
+                        '-- setsid',
                         'ps -o command -C ovsdb-server --no-headers -ww',
                         '|| true'))
         ovsdb_server_cmd = cls.proxy_host_client.exec_command(cmd)