Slapcontainer: minor fix

Some slapos.cfg don't use interface_name but bridge_name in order
to specify the bridge.

This allow slapcontainer to support both.

slapos/container/__init__.py

@@ -50,7 +50,10 @@ def run(args):
 
     partition_amount = slapos_conf.getint('slapformat', 'partition_amount')
     partition_base_name = slapos_conf.get('slapformat', 'partition_base_name')
+    try:
         bridge_name = slapos_conf.get('slapformat', 'interface_name')
+    except ConfigParser.NoOptionError:
+        bridge_name = slapos_conf.get('slapformat', 'bridge_name')
     instance_root = slapos_conf.get('slapos', 'instance_root')
     partition_base_path = os.path.join(instance_root, partition_base_name)
     partition_list = ['%s%d' % (partition_base_path, i)