Create the "bridge" group on test inventory

The scheduling_group of the obfsvpn service is set to 'bridge' but
this does not exist in the test environment, which results in a
mysterious float error:

[WARNING]:  * Failed to parse /builds/leap/container-
platform/lilypad/build-375077/config.yml with float plugin: 'bridge'

(which is actually a KeyError bubbling up from the inventory plugin).