Add manual default route creation for Ubuntu case as well

ISSUE: Ubuntu affected only.
For interface with gateway defined (/etc/network/interfaces.d/ifcfg-fooX.YYY), 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 as well create default route by manually adding it into routing table, after interface'
configuration and down&up operation has been completed.

We should also ensure right apply order as well (L3_if_downup <||> -> Defaultroute[$def_gateway])

Fixes: #PRD-2023
Depends on: #PRD-1716

Signed-off-by: Bogdan Dobrelya <bogdando@mail.ru>