Arista VRF information

Arista has a vendor MIB for VRF information: ARISTA-VRF-MIB, shown at https://www.arista.com/assets/data/docs/MIBS/ARISTA-VRF-MIB.txt

Hence we implement get_my_vrfs()

Here we read the table for aristaVrfEntry, and get VRF information. This is parsed in _parse_mib_arista_vrf_entries()

We then read aristaVrfIfMembership for VRF member interfaces, and parse this in _parse_mib_arista_vrf_members()