Add functional tests to verify ovs_lib VXLAN detection

This commit adds a functional test to verify host VXLAN support. It compares
the results of this functional test with the logic in ovs_lib to ensure both
report the same values.

Closes-Bug: #1301101

Change-Id: Id91c755d762bea134cc451952d0f13d64576663a
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"