Only open the session file for reading.

I got into a situation where the get_session method hung.

03:02:06.665541153Z 77794 DEBUG manage [health] node.is_primary end: True
03:02:06.665600545Z 77794 DEBUG manage [health] consul.renew_session start
03:02:06.665630532Z 77794 DEBUG manage [health] consul.get_session start
check.mysql-primary timeout after 10s: '[/usr/local/bin/manage.py health]'
03:02:16.259794109Z check.mysql-primary.kill

bin/manager/discovery.py

@@ -68,7 +68,7 @@ def get_session(self, key=SESSION_NAME, ttl=SESSION_TTL,
         if not cached:
             return self.create_session(key, ttl)
         try:
-            with open(on_disk, 'r+') as f:
+            with open(on_disk, 'r') as f:
                 session_id = f.read()
         except IOError:
             session_id = self.create_session(key, ttl)