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.