Escape message when using JSON format for the logs

[hush-hush]

What does this PR do?

Escape message when using JSON format for the logs

This fix broken JSON output like:

{"agent":"core","time":"2019-08-12 16:58:46 MDT","level":"ERROR","file":"pkg/collector/runner/runner.go","line":"298","func":"work","msg":"Error running check max: [{"message": "ERROR", "traceback": "Traceback (most recent call last):
  File \"/home/maxime/dev/go/src/github.com/DataDog/datadog-agent/venv/local/lib/python2.7/site-packages/datadog_checks/base/checks/base.py\", line 532, in run
    self.check(instance)
  File \"/home/maxime/dev/go/src/github.com/DataDog/datadog-agent/bin/agent/dist/max.py\", line 8, in check
    raise Exception(\"ERROR\")
Exception: ERROR
"}]"}

to:

{"agent":"core","time":"2019-08-12 17:04:25 MDT","level":"ERROR","file":"pkg/collector/runner/runner.go","line":"298","func":"work","msg":"Error running check max: [{\"message\": \"ERROR\", \"traceback\": \"Traceback (most recent call last):\\n  File \\\"/home/maxime/dev/go/src/github.com/DataDog/datadog-agent/venv/local/lib/python2.7/site-packages/datadog_checks/base/checks/base.py\\\", line 532, in run\\n    self.check(instance)\\n  File \\\"/home/maxime/dev/go/src/github.com/DataDog/datadog-agent/bin/agent/dist/max.py\\\", line 8, in check\\n    raise Exception(\\\"ERROR\\\")\\nException: ERROR\\n\"}]"}

[albertvaka]

👍