Revert "Changed opening last id file - it's now automatically created even when only reading"

Brown paper bag commit

This reverts commit 1fc362c3.

warden3/warden_client/warden_client.py

@@ -448,7 +448,7 @@ class Client(object):
         if not idf:
             return None
         try:
-            with open(idf, "w+") as f:
+            with open(idf, "r") as f:
                 id = int(f.read())
         except (ValueError, IOError) as e:
             Error(message="Reading id file \"%s\" failed, relying on server" % idf,