proftpd: Add ftp user creation

In order to remove '/home/ftp' and the ftp user from the 'skeleton'
package, we need to add the creation of the ftp users to the package so
it still work out of the box (with an anonymous user).

Signed-off-by Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>