Add validator for max path length

On some operating systems there is a limit on the path length. Allow
project owners to configure a maximum path length, so that the project
stays cloneable on such operating systems.

Change-Id: I0d7d72c9a71f6064f2f68e57bde2707d65bfa9e1
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>