2. Support XML config placeholders
3. Add more tests to cover all kinds of cases
4. Update apollo demo to adapt for auto update functionality

1. Enable config cache to improve config service performance, just set
config-service.cache.enabled to true in ServerConfig
2. Normalize the namespace name in case the one provided has character
case issue

load config file(apollo-env.properties) from working directory first, so that it behaves the same as spring boot config files