Renaming in WalkVersionsMixin

Rename _walk_versions(), _migrate_up(), _migrate_down()
and mark them deprecated as they will be removed in few
releases. These functions will be the part of public API
as this class will be used for migrations in other
OpenStack projects, so there is no need to make them
'private'.

Change-Id: Ic9358445e60a0dd43a5900e8bda7b12f2bebf679
Merge "Renaming in WalkVersionsMixin"