Support MTU configuration for LibvirtHybridOVSBridgeDriver

Allow other MTU configuration for the tap device and the veth pair
then MTU 1500 when LibvirtHybridOVSBridgeDriver is used.

Related-Bug: #1260697
Change-Id: I964eb263e514866ffd15a50005296ee85135a9f5
unittest: Add testcases for device MTU config

Closes-Bug: #1260697
Change-Id: I964eb263e514866ffd15a50005296ee85135a9f5