Give warning when ambiguous 2-part name (`a/b`) is used

[seanyeh]

This would avoid any confusion whether it refers to <project>/<environment> or <org>/default/<environment>
We should recommend to use the 3-part <org>/<project>/<environment> naming

[LouisLau-art]

Opened PR: #617

This adds a warning when a 2-part environment ref like a/b is potentially ambiguous (when a matches an org the user belongs to), and recommends using the unambiguous 3-part form <org>/<project>/<environment>.