*: change interface structure, from vrf_id to vrf

*: change interface structure, from vrf_id to vrf

Field vrf_id is replaced by the pointer of the struct vrf *.
For that all other code referencing to (interface)->vrf_id is replaced.
This work should not change the behaviour.
It is just a continuation work toward having an interface API handling
vrf pointer only.

some new generic functions are created in vrf:
vrf_to_id, vrf_to_name,

a zebra function is also created:
zvrf_info_lookup

an ospf function is also created:
ospf_lookup_by_vrf

it is to be noted that now that interface has a vrf pointer, some more
optimisations could be thought through all the rest of the code. as
example, many structure store the vrf_id. those structures could get
the exact vrf structure if inherited from an interface vrf context.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>