This issue aims to support register one or more ExternalCatalog (which is referred in https://issues.apache.org/jira/browse/FLINK-5568) to TableEnvironment. After registration, SQL and TableAPI queries could access to tables in the external catalogs without register those tables one by one to TableEnvironment beforehand.
We plan to add two APIs in TableEnvironment:
1. register externalCatalog
2. scan a table from registered catalog and returns the resulting Table, the API is very useful in TableAPI queries.