This PR adds an interface for controller API client, it allows us to
implement mock API client for unit testing with ease and we are
able to generate spec file without accessing the real Fission server.
Before this PR, CLI functions fatal out when encountering error
instead of returning it. Such behavior makes it hard to reuse
the functions nor writing unit tests. This PR aims to make functions
return errors instead of error out.
The utility function GetMetadata uses wrong flag text to get the resource namespace and could cause the wrong results returned from the API server. This PR changes the function signature that allows users to pass in the flag text in order to get the correct value.
