Update Dockerfile to disable some redundant log messages

[AlvaroVega]

Default log levels for stein:
https://docs.openstack.org/keystone/stein/configuration/samples/keystone-conf.html

#default_log_levels = amqp=WARN,amqplib=WARN,boto=WARN,qpid=WARN,sqlalchemy=WARN,suds=INFO,oslo.messaging=INFO,oslo_messaging=INFO,iso8601=WARN,requests.packages.urllib3.connectionpool=WARN,urllib3.connectionpool=WARN,websocket=WARN,requests.packages.urllib3.util.retry=WARN,urllib3.util.retry=WARN,keystonemiddleware=WARN,routes.middleware=WARN,stevedore=WARN,taskflow=WARN,keystoneauth=WARN,oslo.cache=INFO,oslo_policy=INFO,dogpile.core.dogpile=INFO

we need to include at least:
keystone.server.flask.application=ERROR

[AlvaroVega]

remove several redundant logs like:
2021-02-25 12:43:21.967 30704 WARNING keystone.server.flask.application [req-40d56104-db96-4326-83a5-ae50ccf0c54c 91fe84a8314d42599d121ac8fd5d39a2 - 6af33733653843139af7b59cb18ccfd4 6af33733653843139af7b59cb18ccfd4 -] You are not authorized to perform the requested action: identity:list_domains.: ForbiddenAction: You are not authorized to perform the requested action: identity:list_domains.

[fgalan]

LGTM