Add private key to docker/tests to access OSM instance

Some of the tests require access to OSM instance in order
to run some commands from there. Those commands could be run
using ssh, but a private key is required to be able connect to
that OSM instance.

This change gets the private key corresponding to the public key in
OSM instance and mounts it in docker/tests so that the container can
access to OSM instance via ssh and run commands there.

Change-Id: Id12ede582d5e519afbc41647b90d192b80cf8fcf
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>