slaprunner: parameterize the shared folder

In test nodes we put the software_root folder out of the webrunner so we
can keep it for the following tests. Except that the shared folder has
been introduced by default recently, and it is still inside the webrunner.

Thus, between 2 tests the webrunner is deleted, so the shared parts are too,
but not the SR folder (which is marked as completed). Then in the successive tests
the software release fails to build, or the insances fail to isntanciate.