PRD-1716: Fix default route for Centos

Note: https://github.com/Mirantis/fuel/pull/662 depends on this one

ISSUE: Centos affected only.
For system wide configured gateway (/etc/sysconfig/network), there is no ensure 
it would has been applied as a default route. It will be applied only after node's 
reboot or network restart.

SOLUTION: 
We should avoid using "heavy" operations like 'service network restart', thus, 
we should create default route by manually adding it into routing table while 
its config stage (adding GATEWAY to /etc/sysconfig/network) is being executed as well.

We should also ensure right apply order as well (L3_if_downup <||> -> Defaultroute[$def_gateway])