PRD-2023: Fix default route for Ubuntu

Note: depends on https://github.com/Mirantis/fuel/pull/641

Assumes never put several interfaces to single L2 broadcast domain, if one of them is dhcp enabled, otherwise:
ISSUE:
For vlan/bond enabled interfaces, multiple or wrong default routes might have been created
on start-up time, thus, preventing desired gateway x.y.z.t would have been
added for default route on its static subinterface(vlan/bond slave) start-up.

F.e. for eth0 auto with dhcp and its eth0.100 subint with gateway x.y.z.t defined:
1) eth0 starts up
2) default route have created from dhcp-defined gateway f.g.h.i via eth0 (or ethZ, if it shares the 
same L2 broadcast domain with eth0)
3) eth0.100 starts up
4) gateway x.y.z.t ignored as well ...

To avoid this, you can add pre-up hooks for every main/slave interface to erase existing
default route (if any) configured via this or any other main interface, but that would be an ugly solution.
_____________________________________________________________________________________
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])