test: improve db_replicator coverage

This patch adds a test for ReplicatorRpc.complete_rsync()
and complete extract_device() coverage.

test_extract_device:
  test the case the parameter is invalid

test_complete_rsync_with_bad_input:
  ensure the use of invalid parameters return a 404 erro

test_complete_rsync:
  validate the returned code in case of success

Change-Id: I59e0d26a1efe59d8beff1e81c2a7edc6de0872e9
Merge "test: improve db_replicator coverage"