security_group: Add head -1 to get_bridge_physdev (#3648)

We've encountered a corner case where bridge -o link show returned two lines per bridge instead of one. get_bridge_physdev in security_group.py returned bond0.701\nbond0.701.
Although this may very well be something on the hypervisor, we should limit the lines returned.
I therefore added a mere | head -1 to the function.

Author: Rene Diepstraten

scripts/vm/network/security_group.py

@@ -147,7 +147,7 @@ def split_ips_by_family(ips):
 
 
 def get_bridge_physdev(brname):
-    physdev = execute("bridge -o link show | awk '/master %s / && !/^[0-9]+: vnet/ {print $2}'" % brname)
+    physdev = execute("bridge -o link show | awk '/master %s / && !/^[0-9]+: vnet/ {print $2}' | head -1" % brname)
     return physdev.strip()