Convert _poll_unconfirmed_resizes to use Migration object

We can now call the compute api directly for confirm_resize, so the
conductor method 'compute_confirm_resize' has been deprecated.

This was also the last call to conductor's:
'migration_get_unconfirmed_by_dest_compute'

so it, too, has been deprecated.

Related to blueprint unified-object-model

Change-Id: I57a4898f14d35e97d3a9ed8fd2a99f157c4111a7
Merge "Convert _poll_unconfirmed_resizes to use Migration object"