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

1. add aop to log spring jpa repository transaction
2. use gson to improve performance
3. specify http message converters for rest template
4. specify content type request header for release operation in portal

1. Add a release key generator to generate human-readable release keys
2. Add retry policy for client long polling
3. Move eureka url config to database

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