ms_driver: Switch to process.Process from subprocess

Use a single tmp/rundir for all these instances. In the next step
we can create separate directories.

Change-Id: Icf4d4e161ac4283a63ed4e0745b375e7e6a25004
diff --git a/src/osmo_ms_driver/__main__.py b/src/osmo_ms_driver/__main__.py
index 265ce47..0e3611b 100644
--- a/src/osmo_ms_driver/__main__.py
+++ b/src/osmo_ms_driver/__main__.py
@@ -83,7 +83,7 @@
     ev_server.listen(loop)
 
     # Just a single test for now.
-    options = BinaryOptions("virtphy", "mobile", None)
+    options = BinaryOptions("virtphy", "mobile", os.environ)
     test = MassUpdateLocationTest("lu_test", options, args.num_ms, cdf, ev_server, tmp_dir)
     atexit.register(test.stop_all)