Bugfix: support for nodes with private LAN only

This file unnecessarily uses "ifdown" and "ifup" instead of "service network restart" to apply configuration changes. Because of this, hosts configured to only allow SSH access over eth1 (private LAN) become inaccessible following "vagrant up" because ifdown terminates the only accessible SSH path. 

The above fixes this problem on EL6/RHEL hosts while still achieving the intended goal of allowing Vagrant to manage all interfaces.

Why is this important? Many advanced network configurations, particularly those in corporate environments, will include hosts NAT'd behind a firewall with either (A) No publicly configured interface on eth0 (due to PaaS/cloud requirements or convention), or (B) SSH access restricted to eth1 and only accessible via a VPN. This fix allows such environments to be perfectly replicated while in development.