Ensure default route is purged before gateway-defined subint will be UP

ISSUE: Ubuntu affected only.
For vlan enabled interfaces with dhcp, wrong default route would have
been created on start-up time, thus, preventing gateway x.y.z.t
would have been added for default route on subinterface(vlan) 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 for defined by dhcp gateway f.g.h.i
3) eth0.100 starts up
4) gateway x.y.z.t ignored as well ...
5) ???
6) profit

SOLUTION: add pre-up hook for gateway-defined subinterface to erase existing
default route, if any. That will ensure there is no default route exists
after its main interface start-up has been finished.

Fixes: #PRD-2023

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