set-project-parent --children-of: Fix getting parent for level 1 projects

For direct child projects of the 'All-Projects' project the name of the
parent project was incorrectly retrieved if the parent name was not
explicitly stored as 'All-Projects' in the project.config file.

Change-Id: Ic9aab7f96c04bac21778f8c93bc5cb5056161ced
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>