Remove device_exists in LinuxBridgeManager

LinuxBridgeManager is adding a device_exists method instead of
using the one in ip_lib as the rest of the codebase

Change-Id: Ibeb95be76a1e1b20c69841e708c2a2cbf1cbe05a
Closes-Bug: #1302463
Merge "Remove device_exists in LinuxBridgeManager"