Add support to run attestation demo

This test case tests attestation using keybroker server/app.

Keybroker-server/app are used from
https://github.com/veraison/keybroker-demo/

.

Keybroker-server is run on FVP docker and Keybroker-app is run on realm
guest.

Signed-off-by: Jagdish Gediya <jagdish.gediya@arm.com>