Remove shell parameter from the call of subprocess_popen

Remove shell parameter from subprocess_popen, [1] removes
shell reference in the above method in Neutron to fulfill
bandit B604 requirement.

[1]: https://review.opendev.org/c/openstack/neutron/+/915467

Change-Id: I6866f66e1ee74b28fbe28ec30c11126d00652c2d

neutron_vpnaas/services/vpn/common/netns_wrapper.py

@@ -56,7 +56,7 @@ def execute(cmd):
     cmd = list(map(str, cmd))
     LOG.debug("Running command: %s", cmd)
     env = os.environ.copy()
-    obj = utils.subprocess_popen(cmd, shell=False,
+    obj = utils.subprocess_popen(cmd,
                                  stdin=subprocess.PIPE,
                                  stdout=subprocess.PIPE,
                                  stderr=subprocess.PIPE,