Check if bridge exists and make sure it's UP in ensure_bridge

Introduce _bridge_exists_and_ensure_up that checks if the bridge
exists and if it's DOWN sets it UP

Change-Id: Ide9d2bb04016cadb347b00cadbee7007e61bd01e
Closes-bug: #1282741
Merge "Check if bridge exists and make sure it's UP in ensure_bridge"