Fix iptable rules in ubuntu 20 for bridge name (#5318)

In ubuntu20 the interface name contains @ synbol and
because of that even the iptable rules for brdige name
contains this symbol which causes ping issues.
Remove the @ symbol from iptable rule to fix the issue

Co-authored-by: Rakesh Venkatesh <rakeshv@apache.org>

Author: ravening

scripts/vm/network/security_group.py

@@ -185,7 +185,7 @@ def destroy_network_rules_for_nic(vm_name, vm_ip, vm_mac, vif, sec_ips):
         logging.debug("Ignoring failure to delete ebtable rules for vm: " + vm_name)
 
 def get_bridge_physdev(brname):
-    physdev = execute("bridge -o link show | awk '/master %s / && !/^[0-9]+: vnet/ {print $2}' | head -1 | cut -d ':' -f1" % brname)
+    physdev = execute("bridge -o link show | awk '/master %s / && !/^[0-9]+: vnet/ {print $2}' | head -1 | cut -d ':' -f1 | cut -d '@' -f1" % brname)
     return physdev.strip()