Add env variable to define location of OSM private key 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 adds an env variable to define the location of the private key to
access OSM instance. That env variable should be used by the tests requiring
access to OSM instance via ssh to run commnads.

Change-Id: Ifb25406d3145f6af862f071cb6bfb90ef0513e7f
Signed-off-by: garciadeblas <gerardo.garciadeblas@telefonica.com>