Configurable placement during resize

Currently OpenStack has a boolean "allow_resize_to_same_host" config option that constrains placement during resize. When this value is false, the ignore_hosts option is passed to the scheduler. When this value is true, no options are passed to the scheduler and the current host can be considered. In some use cases - e.g. PowerVM - a third option of "require same host' is desirable.

This blueprint will deprecate the "allow_resize_to_same_host" config option and replace it with "resize_to_same_host" that supports 3 values - allow, forbid, require. Allow is equivalent to true in the current use case (i.e. not scheduler hint, current host is considered), forbid to false in current use case (i.e. the ignore_hosts scheduler hint is set), and require forces the same host through the use of the force_hosts scheduler hint.