Pass migration_ref when when auto-confirming

The _poll_unconfirmed_resizes() task in compute/manager calls back to
compute_api to confirm resizes. This triggers a db lookup for the
migration, which is not allowed by no-db-compute. Since we already have
the migration, pass it through to avoid the lookup.

Since this doesn't change rpcapi (and thus, RPC clients should not
know about this optional paramter), we don't need to bump the RPC
version here.

Fixes bug 1152792

Change-Id: I4bd3b0c88968ed7f64e7df0afa12e83009ee8ccf
Merge "Pass migration_ref when when auto-confirming"