cyclades: Link serial to resource in handler

Move code that attaches a newly created serial from issue_commission
to handle_resource_commission; the latter is responsible to clean up
previous serials and thus to attach the new one.
cyclades: Accept serial before actually building vm

Since the vm is written in the DB, we accept the corresponding serial
before sending the build job to the backend. This is to avoid
rejecting the serial when the job fails, as is the case with other
kind of actions.
cyclades: Don't reuse a resource object after transaction ends

Accept/reject a serial without touching the respective resource when
we have explicitly committed or rolled back a transaction, because
that would risk overwriting a resource locked in another transaction.
cyclades: Mark vm build serial to accept in advance

When creating a vm, mark the serial as acceptable along with
committing the vm to the db. This makes sure the commission will be
eventually accepted, no matter what fails in the next steps.