run_armds_script.sh: Ask user which DSTREAM to connect to

Previously the script to run the EL3 payload on Juno would connect to
the first DSTREAM it detected, without even asking the user's consent.

Change-Id: I1586957094cc3247c1dadf0adac263af0256e03d
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>