Refactor Keystone to use unified logging from Oslo

Modifications to use log from /keystone/openstack/common/log.py instead
of /keystone/common/logging.py. This change also includes some
refactoring to remove the WriteableLogger class from common/wsgi.py
since that is already included in the unified logging sync from Oslo.
This also moves fail_gracefully from /keystone/common/logging.py to
service.py as it is only used within that module.

blueprint unified-logging-in-keystone

Change-Id: I24b319bd6cfe5e345ea903196188f2394f4ef102
Merge "Refactor Keystone to use unified logging from Oslo"