Simplified and stabilized ryu launch

The rest app only considered a single fixed filename rather than simply
relying on the existing loading mechanism.
For better clarity some variable names were renamed and logging now
outputs the full ryu-manager command.

Change-Id: I35ac86159880acc71d460df7db95e1b244c82eb3
Signed-off-by: schillinge <ablu@mail.uni-paderborn.de>