Fix update_device_up method of linuxbridge plugin

Also add unit tests covering update_device_up and update_device_down
methods

(cherry picked from commit: 01e45596fae2818b41c76b41bef46e86c5d33231)

Change-Id: I97f2f9249b684aa5350b3f0621754543e80bec70
Closes-Bug: #1241602