tests: Use exponential backoff when ssh'ing to save QEMU cycles.
tests: Use exponential backoff when ssh'ing to save QEMU cycles.

Handshakes are expensive and tend to pile up.

Changelog: None

Signed-off-by: Kristian Amlie <505e66ae45028a0596c853559221f0b72c1cee21@northern.tech>

GregorioDiStefano/integration,GregorioDiStefano/integration,pasinskim/integration,pasinskim/integration,pasinskim/integration