Restore heat.db.sync and add Deprecated message.

This unpythonic deprecation method is used because this module was only
ever meant to be used from the CLI as 'python -m heat.db.sync'. The
module should be removable in Havana+1.

Fixes bug #1185220

Change-Id: I9943e5c35d9578f899fae1d11b818a2a5a9167c0