chore(config): move Sanitize and Force check to boot process

Make Load function just load parameters. So etcd instance could
use Config struct to start service.
refactor(tests/server_utils): use etcd instance

Remove duplicated etcd start code.
chore(cors): more explanation for flush in ServeHTTP
Merge pull request #775 from unihorn/84

refactor(tests/server_utils): use etcd instance