Reuse Salesforce session id

- Session file
- One worker at a time has ability to lock session file,
  auth and write new session id
- Other workers waits and reuse the session id
- Workers compare their session id with the one in file

Change-Id: I371b0a7172a5b312a2ae2aef96d02505a1823f86
Related-bug: PROD-29347 (PROD:29347)
diff --git a/sf_notifier/helpers.py b/sf_notifier/helpers.py
index 295c566..e9c221b 100644
--- a/sf_notifier/helpers.py
+++ b/sf_notifier/helpers.py
@@ -13,10 +13,18 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import os
+
 
 RESOLVED_STATUSES = ('UP', 'OK', 'resolved')
 
 
+def create_file(file_path):
+    if not os.path.exists(file_path):
+        with open(file_path, 'w+'):
+            return file_path
+
+
 def _format_subject(alert):
     subject = alert['annotations']['summary']
     host = alert['labels'].get('host')