Periodic task _poll_unconfirmed_resizes can now use slave db

This allows the _poll_unconfirmed_resizes periodic task to use the
slave db for reads if the deployment has a slave configured. For
slaveless deployments behavior remains unchanged.

This commit advances the MigrationList object to version 1.1

Change-Id: I10bf729e92dc599819ef74a97a834c7f2df5b162
Partially-implements: blueprint periodic-tasks-to-db-slave
Merge "Periodic task _poll_unconfirmed_resizes can now use slave db"