The rkt driver currently executes run and asks that the pod UUID is
written to a file that is then polled for changes for up to five
seconds. Many container fetches will take longer than this, so this
method will often not be able to track the pod UUID reliably.

To avoid this problem, rkt allows pods to be first prepared, which will
return their UUID, and then run as a second invocation.

Here we convert the rkt driver's Start method to use this method
instead. This way, the UUID will always be tracked correctly.

Currently the rkt driver does not expose a DriverNetwork instance after
starting the container, which means that address_mode = 'driver' does
not work.

To get the container network information, we can call `rkt status` on
the UUID of the container and grab the container IP from there.

For the port map, we need to grab the pod manifest as it will tell us
which ports the container exposes. We then cross-reference the
configured port name with the container port names, and use that to
create a correct port mapping.

To avoid doing a (bad) reimplementation of the appc schema(which rkt
uses for its manifest) and rkt apis, we pull those in as vendored
dependencies. The versions used are the same ones that rkt use in their
glide dependency configuration for version 1.28.0.

Since we don't actually show these errors anywhere, just discard them
right away.

To test that the rkt driver correctly sets a DriverNetwork, at least
when a port mapping is requested, we amend the
TestRktDriver_PortsMapping test with a small check.

If the rkt driver cannot get the network status, for a task with a
configured port mapping, it will now fail the Start() call and kill the
task instead of simply logging. This matches the Docker behavior.

If no port map is specified, the warnings will be logged but the task
will be allowed to start.

The changes introduces in #3256 require at least rkt 1.27.0 because of
a bug in the JSON output of `rkt status` in previous versions.

Here we upgrade all references to rkt's minimum version, and also make
travis and vagrant use this version when running tests.

Finally we add a CHANGELOG notice.

The network status poll loop for the rkt drivers `Start` method was a
bit messy, and could not display the last encountered error. Here we
clean it up.

The network status poll loop will now report any networks it ignored, as
well as a no-networks situations.

The current Travis setup scripts copy in rkt, but do not set up a
default container network.

Here we copy the container network setup over from the vagrant setup
scripts.

The rkt port mapping test currently starts redis with --version, which
obviously makes redis exit again almost immediately. This means that the
container exists before the network status can be queried, and so the
test fails.

If the container dies before the network can be read, we now ignore the
error coming out of the network information polling loop. Nomad will
restart the task regardless, so we might be masking the actual error.

The polling loop for the rkt network information, inside the `Start`
method, was getting a bit unwieldy. It's been refactored out so it's not
a seperate function.