Add support for additional disks, big nodes, update channel and Parallels

[dab-q]

Sometimes it can be desirable to configure additional disks,
more memory and more CPUs to the nodes. In a constrained
environment with a multi-node cluster, there might not be enough
resources to assign larger memory/cpu resources to all the nodes.
These changes support two node configurations, as well as
configuring all nodes with additional virtual disks.

Create big nodes

• In addition to the normal nodes, you can
  also add additional nodes with a larger
  memory/cpu configuration.

  $num_big_instances = 1
  $vm_big_memory = 8192
  $vm_big_cpus = 2

Additional disks

• Each node can be configured with additional disks.
  $num_data_disks = 3
  $data_disk_size = 10 # GBytes

Specify the coreos update channel:
$update_channel = "alpha"

Add .virtualbox/ to .gitignore

Add Parallels support

• Set the memory size and number of cpus for Parallels,
  as well as setting the download path.

• Change Vagrant file config.vm.provider order
  so that VirtualBox is first, which makes it
  the default provider (assuming it works), and
  the util.rb reflects that assumption. In general,
  setting VAGRANT_DEFAULT_PROVIDER is the best way
  to be deterministic about what provider is used.

[dab-q]

We are using coreos-vagrant for a developer environment for a https://github.com/rook/rook installation, and we need additional disks on each VM for Rook/Ceph to consume. We are also running some additional software that has larger memory and CPU requirements, but we only need one node in the cluster to be able to support it, so having several 1CPU/2GB nodes and one 2CPU/6GB nodes works well in a constrained resource environment like a MacBook Pro laptop.