Bump oslo.utils minimum version to 3.18.0

This version introduces some useful methods from existing projects to
oslo.utils. These methods are:
*flatten_dict_to_keypairs
*_Event
*Event

At the same time, it fixes a performance issue in Oslo Messaging:
change I5c211092d282e724d1c87ce4d06b6c44b592e764.

upper constraint has been updated in https://review.openstack.org/#/c/393212/.

Change-Id: I459375d7ec4ad151b4d8994e3905681bfa458070

global-requirements.txt

@@ -120,7 +120,7 @@ oslo.reports>=0.6.0  # Apache-2.0
 oslo.rootwrap>=5.0.0  # Apache-2.0
 oslo.serialization>=1.10.0              # Apache-2.0
 oslo.service>=1.10.0                     # Apache-2.0
-oslo.utils>=3.17.0                       # Apache-2.0
+oslo.utils>=3.18.0                       # Apache-2.0
 oslo.vmware>=2.11.0                     # Apache-2.0
 oslo.versionedobjects>=1.13.0            # Apache-2.0
 osprofiler>=1.4.0                       # Apache-2.0