pipeline init command should handle existing workspace

[waynz0r]

The pipeline init command should warn the user if the workspace directory already exists.

[orymate]

It checks for the values file now, because, for example, the user may want to prepare some files to the directory before init.

Mate% banzai cp init                     
INFO[0000] You can create another workspace with --workspace, or run `banzai pipeline up` to deploy the current one.
workspace is already initialized in "/Users/maat/.banzai/pipeline/default"```