Refactor ensure-bridge for persistance

Updates ensure-bridge so that we use network-scripts and eni files
to configure persistent network configs using the distro provided
mechanism. See docs here:

http://git.openvswitch.org/cgi-bin/gitweb.cgi?p=openvswitch;a=blob;f=rhel/README.RHEL;hb=HEAD
http://git.openvswitch.org/cgi-bin/gitweb.cgi?p=openvswitch;a=blob;f=debian/openvswitch-switch.README.Debian;hb=HEAD

This resolves some problems with the previous approach affecting
lease renewal and machine reboots.

With the new approach:

- tmp config files are written to disk
- the real config files are overwritten if a change is detected
- ifup/ifdown commands are used to start/stop the network interfaces
  if a change has been made
- All of the ip, ovs-vsctl, etc. commands are handled by the distro
  scripts

One more important change is that ensure-bridge expects both
the EXTERNAL_BRIDGE and PHYSICAL_INTERFACE parameters. Previously
the script only required EXTERNAL_BRIDGE. We don't have a use case
for using ensure-bridge without a physical interface so I just made
both required since this made the implementation simpler.

Change-Id: Ib775598d3bdd6f1ccecb283f47d10b92aaf0bd35
Closes-bug: #1272969
Merge "Refactor ensure-bridge for persistance"