Add the support of VM name auto-generation

[henrysher]

It is quite useful when we have multiple machines for one vsphere template and bootstrap each VM differently.

Currently we just can configure only one VM name for each template. So it would be nice to support VM name auto-generation, or add a prefix on the auto-naming.
Thus I could use multiple machine config in Vagrantfile to quick launch VMs.
Like:

  config.vm.define "slave0" do |slave0|
    slave0.vm.box = "dummy"
  end
  config.vm.define "slave1" do |slave1|
    slave1.vm.box = "dummy"
  end

[agrauch]

Merged with pull request #35