DHCP renewal on Debian/Ubuntu after setting hostname

On Debian and Ubuntu guests, when the new hostname of the guest box is set, the DHCP client (isc-dhcp-client) won't renew its DHCP lease with the new hostname. Executing "ifdown -a; ifup -a" to reinitialize all auto interfaces fixes this issue.

Furthermore, as vagrant will now actually wait until a DHCP lease is acquired, it is guaranteed that the the correct domainname is set from DHCP before calling other provisioners like puppet, that rely on it. This fixes puppet/facter sometimes failing to find the fqdn fact on an LXC guest when the DHCP server is responding too slow.