make get-pip.py download failures more clear

curl dying ends up being a really unclear failure condition, and
hard to fingerprint in the gate. We should make this much more
explicit when we die.

Also, don't trust the upstream filename, because all the rest of
our logic would break if it changes anyway.

Change-Id: Ibc2a96b33471d24c597af0d7af896fb10523156f
a more proper default nic selection criteria

look at all interfaces that ip link shows as being up, then filter
them for ^e or ^p devices. Should hold even after the bridges get
built.

Change-Id: If6b1e623f9d146c2430dc8a4a007997290b90846