Added support for sending the hostname in a DHCP request for bridged adapters in RedHat guests.

Usage:

Vagrant.configure("2") do |config|
  config.vm.box = "CentOS 6.3 Minimal"
  config.vm.hostname = "host.example.dev"
  config.vm.network :public_network, :send_hostname_in_dhcp_request => true
end