added deprecated.warn helper method
added deprecated.warn helper method

provide a convenience method for indicating in code that a config
option the operateor will have to deal with exists in their environment.

Change-Id: I17b0c120d54b1db75c6bb29d107e4a15a5202e76

citrix-openstack-build/oslo.versionedobjects,openstack/oslo.versionedobjects