Prefix interface names with 'veth' so NetworkManager ignores them

[bboreham]

Fixes #1783

[dpw]

(NM behaviour seems to have changed in ubuntu 15.10. I sometimes see it produce notifications for the 'veth' prefixed netdevs created by docker. I'm not sure what the story is.)

[awh]

Once #1758 is merged, the names veth$BRIDGE and veth$DATAPATH become available if that's preferable (see #1783 (comment))

[bboreham]

I went with veth$BRIDGE and vethwe$DATAPATH; the latter so it has a vague reference to "weave"

[awh]

One has a hypen and the other doesn't...

[bboreham]

I know; we're limited in the number of characters we can have.

[bboreham]

However I found I could squeeze exactly one more in.