check if vm is not created by monitoring

There are situation when between spawning VM on compute site and registering
this machine inside atmosphere DB VM monitoring is triggered. As a conclusion
VM with duplicated `id_at_site` is create, which causes
`Unable to assign VM to appliance` error. Here we are fixing this issue by
checking if VM exists in database or initializing it
(`find_or_initialize`).

@nowakowski This change is deployed into `vph-dev` - please test it and report back