Ensure default route will exist after netconfig stage is done

Fixes: #PRD-1716

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>
Put default route after interface creation

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])

Fixes: #PRD-1716

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>
Use ip route replace instead of route add

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>
Merge pull request #641 from bogdando/fix_prd1716

PRD-1716: Fix default route for Centos