Usage of 0.0.0.0 exposes API endpoint to all networks and can cause
unexpected leakage. This change updates the default to use more
restricted address (127.0.0.1, which is the default defined in Watcher
itself) unless the address is given by a user explicitly.
Change-Id: Ibc895cf2b06d3b0c1b2073d8d6aa90d7d6f299d2