Add Cloud Networks capabilities

Allows for the addition of isolated Cloud Networks to
instances. By default, instances will have a public and
ServiceNet network available. ServiceNet can be removed
by passing :no_servicenet to the network configuration.

config.vm.provider :rackspace do |rs|
  rs.network '443aff42-be57-effb-ad30-c097c1e4503f'
  rs.network '5e738e11-def2-4a75-ad1e-05bbe3b49efe'
  rs.network :no_servicenet
end
add config validation tests
add config validation for URLs
Merge pull request #39 from maxlinc/validation

URL validation
Use alternate syntax for disabling ServiceNet

Instead of passing :no_servicenet to rs.network, the following
syntax should be used:

config.vm.provider :rackspace do |rs|
  rs.username = "mitchellh"
  rs.api_key  = "foobarbaz"
  rs.network '443aff42-be57-effb-ad30-c097c1e4503f'
  rs.network :service_net, :attached => false
end
Merge pull request #37 from DavidWittman/cloud-networks

Add Cloud Networks capabilities