Refactor discovery to use invokeInWorkspace 

This is a major set of changes and should be released as a minor version.

 - Refactor `invokeInWorkspace()` to support all `dataSource` methods
 - Implement `getSchema()` and `discoverModelDefinition()` using `invokeInWorkspace()`
 - Add new `dataSourceDef.createModel()` to create model definitions + configs using a discovered model definition from `discoverModelDefinition()`

/to @bajtos @raymondfeng 
/cc @seanbrookes 
This message was not part of the execution error message, it was printed by loopback to stdout as a warning when an unknown connector was ignored. 

Thus this condition will be never true unless I am missing some other change.