Fix find_user() method to deal with ambiguous groups

Add error_ambiguous_user() method
Improve user arguments allowing to specify input type (id or name)

Refactor list_users() methods to call find_user(), find_user_by_id() and/or
find_user_by_name() depending on the input.
Fix double whitespace
Merge pull request #1053 from sbesson/cli_finduser

CLI: Improve user arguments allowing to specify input type