Add the support of VM launching in parallel

[henrysher]

It takes much time when we need to clone multiple VMs from a template in a queue.
If we could support VM launching in parallel, it would be great!

[marnovdm]

I use multiple separate vagrant projects (which can be 'vagrant up'-ed at the same time) and linked clones to speed up initial VM creation times, but I agree it would be great if this was supported!

[marnovdm]

This works really well and greatly speeds up provisioning, thanks for this!

One minor request would be to also destroy the VMs in parallel if possible? I noticed that still happens sequentially, and for testing we deploy and remove a lot of VMs.