vlan: fix format string for 1000 bridges/vlans test
Fix the following:
Traceback (most recent call last):
File "/usr/local/lib/python3.6/site-packages/behave/model.py", line 1329, in run
match.run(runner.context)
File "/usr/local/lib/python3.6/site-packages/behave/matchers.py", line 98, in run
self.func(context, *args, **kwargs)
File "features/steps/connection.py", line 378, in add_bridges_vlans_range
'add connection %s failed: %s' % (result['error'])
TypeError: not enough arguments for format string
Edited by Beniamino Galvani