Add osversion field to CreateVMForm

- vm_osversion is passed as extra variables to the create_vm job
template
- vm_cores/vm_memory/vm_disk_size are also passed as extra variables
instead of saving them as host variables. Saving the variables in the
host when submitting the form forces to update the inventory before to
trigger the job to create the VM. This can be avoided using extra
variables. Furthermore those variables don't need to be in the
inventory, they can be retrieved as facts.

JIRA INFRA-877