Prevent STDERR warnings for metadata get with --default

If you're getting non-existent build metadata with a `--default` via Docker, you end up with the STDERR combined in the output of the default value.

For example, without Docker you can do this:

```shell
output=$(buildkite-agent meta-data get foo --default default-foo)
# Warnings are printed to STDERR
echo $output
#=> "default-foo"
```

But when you use Docker run, Docker smooshes together STDOUT and STDERR into the STDOUT stream of the run command, and you end up with the warning in the output:

```shell
output=$(
docker run \
-e BUILDKITE_BUILD_ID \
-e BUILDKITE_JOB_ID \
-e BUILDKITE_AGENT_ACCESS_TOKEN \
-t --rm \
buildkite/agent:3 \
buildkite-agent meta-data get foo --default default-foo
)

echo $output
#=> "2019-08-16 03:36:28 WARN No meta-data value exists with key `foo`, returning the supplied default "default-foo""
```

A workaround is to use bash to redirect STDERR:

```shell
output=$(
docker run \
-e BUILDKITE_BUILD_ID \
-e BUILDKITE_JOB_ID \
-e BUILDKITE_AGENT_ACCESS_TOKEN \
-t --rm \
--entrypoint '' \
buildkite/agent:3 \
bash -e -c 'buildkite-agent meta-data get foo --default default-foo 2>/dev/null'
)

echo $output
#=> "default-foo"
```

It'd be nice to perhaps not output a warning at all in the case of using `--default`, or if it's kept then an option for ignoring warning logs?

Start with the `buildkite-agent meta-data get --default` entry point and reproduce the behavior through the Docker invocation shown. Determine the intended warning behavior when a supplied default is returned, then verify that captured Docker output matches the agreed behavior.

Written by the indexing model from the issue text.