lxc-net: Rework/cleanup

lxc-net: Rework/cleanup

This updates lxc-net with the following changes:
 - Better recover from crashes/partial runs
 - Better error detection and reporting
 - Less code duplication (use the stop code on crash)
 - Better state tracking
 - Allow for restart of all of lxc-net except for the bridge itself
 - Only support iproute from this point on (ifconfig's been deprecated
   for years)

V2: Use template variables everywhere

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>