Description of problem:

Following the recommendation of keystone.conf 

>> https://docs.openstack.org/keystone/rocky/configuration/samples/keystone-conf.html
>> .. 
>> # Cache backend module. For eventlet-based or environments with hundreds of
>> # threaded servers, Memcache with pooling (oslo_cache.memcache_pool) is
>> # recommended. 

oslo_cache.memcache_pool was enabled for cache backend of keystone service 

>> [cache]
>> backend=dogpile.cache.memcached 

But when the keystone service is started, there is error on keystone.log

>> Error stevedore extenstion could not load oslo_cache.etcd3gw no module named etcd3gw module not found error"

And etcd3gw is not available on keystone kolla container as seen below 

cat /lib/python3.6/site-packages/oslo.cache-1.37.1-py3.6.egg-info/requires.txt

[dogpile]

[dogpile:python_version == "2.7"]
python-memcached>1.55

[dogpile:python_version >= "3.4"]
python-memcached>1.57

[etcd3gw]
etcd3gw>=0.2.0

[mongo]
pymongo!=3.1,>=3.0.2

$ rpm -qa | grep python3 | grep etcd |  wc -l
0

$ pip3 list | grep -i etcd | wc -l
0



Version-Release number of selected component (if applicable):


How reproducible:


Steps to Reproduce:
1. follow the description above to reproduce using RHOSP 16.2.3
2. find the error as the description above 

Actual results:
There is error on keystone log 

Expected results:
there should not be any error log on keystone

Additional info:

I've checked https://github.com/openstack/kolla but python-etcd3gw is missing from openstack base image dockerfile of rhel container image.