arista-veos: enable VARP

But this doesn't seem to work. In fact, once we get the port channel,
nothing seems to work as expected.
arista-veos: make the MLAG work as expected

We need to authorize data VLAN to travel on this MLAG...
arista-veos: disable spanning tree

Our use of native VLAN make MSTP believe there is loop while this is not
the case.
arista-veos: use two different VLAN to Internet to avoid any switching

This should not be necessary, but I am too lazy to test. The rationale
behind this is that EOS may share information with its peer if it
believes that this is the same VLAN. We use VLAN only because it was not
possible to use raw Ethernet interface. However, VLAN are declared as
switchport. We don't really want that.
arista-veos: fix IP of C1/C2/C3 in schema
arista-veos: add some information in README