Deprecate WritableLogger - used for eventlet logging
Deprecate WritableLogger - used for eventlet logging

WritableLogger was used for example in Nova as a bridge to the
eventlet's logging mechanism. Eventlet has been enhanced in
0.17.2 to use python loggers directly, so we don't need this
anymore.

Closes-Bug: #1440773
Change-Id: I505a6e4946db88db4a1e57ed37f43bd203c7ab1f

magic0704/oslo.log,meganjbaker/oslo.log,zzicewind/oslo.log,openstack/oslo.log