Change allegedly running VMs sooner if appropriate

[dberzano]

• Run check_owned_instance() many times, more granularly and use the deployment time as a timeout: if it hasn't been reached, reschedule the check; if it has been reached, terminate the VM.
• Also, do not schedule change_vms_allegedly_running(), but use check_owned_instance() to change it, both on success (i.e. VM is running) and on failure (i.e. timeout reached).