Create core directory to contain most of code not in object classes used by tests

Change-Id: I9aec6c55ccd71894182057d36e0025b69925d314
diff --git a/src/osmo_gsm_tester/run_node.py b/src/osmo_gsm_tester/run_node.py
index 88555a6..e5f7d56 100644
--- a/src/osmo_gsm_tester/run_node.py
+++ b/src/osmo_gsm_tester/run_node.py
@@ -17,7 +17,7 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-from . import log
+from .core import log
 
 class RunNode(log.Origin):