2. Reorganize the feature to provide the ability to inject apollo
configs in Spring Boot bootstrap phase
3. Add more integration tests to cover different scenarios
4. Update apollo-demo to show this apollo bootstrap config feature could
be used with @ConditionalOnProperty

1. specified cluster
2. data center as cluster
3. default cluster

1. Adjust portal jvm memory size
2. Retry if client get empty response from meta server
3. Fix potential npe issues

1. Refactor apollo client code according to  @qmwu2000's comments
2. Add config service integration tests
3. Add some cat events for config service
4. Fix several bugs