security_group: Use execute() function instead of non-existing bash()

In 6233a77 as a part of PR #2432 the
bash() function was replaced by the execute() function.

Somehow this last calling of the bash() function was not caught by testing
and is still in there.

This causes Exceptions to be thrown by the Security Group script.

Signed-off-by: Wido den Hollander <[email protected]>

Author: wido

scripts/vm/network/security_group.py

@@ -1151,8 +1151,8 @@ def getvmId(vmName):
 
 def getBrfw(brname):
     cmd = "iptables-save |grep physdev-is-bridged |grep FORWARD |grep BF |grep '\-o' | grep -w " + brname  + "|awk '{print $9}' | head -1"
-    brfwname = bash("-c", cmd).stdout.strip()
-    if brfwname == "":
+    brfwname = execute(cmd).strip()
+    if not brfwname:
         brfwname = "BF-" + brname
     return brfwname