Merge configuration volume_boot hashes

[Sharpie]

This commit updates the merge behavior of the config object to merge
volume_boot hashes together. This allows defaults to be set for all
VMs that are booting from a volume:

Vagrant.configure('2') do |config|

  # Always delete volumes when VMs are destroyed.
  config.vm.provider :openstack do |os|
    os.volume_boot = {delete_on_destroy: true}
  end

  config.vm.define 'centos1' do |node|
    node.vm.provider :openstack do |os|
      os.volume_boot = {image: 'centos-7.5',
                        size: 20}
    end
  end
end

Prior to this change, the last volume_boot hash specified would overwrite
all configuration set by other provider blocks.

[coveralls]

Coverage remained the same at 87.572% when pulling 9573cb6 on Sharpie:merge-volume-boot into 0e60bd3 on ggiamarchi:master.