Debug mode, see more logs with ?debug=true

[1vn]

Closes #328
In imagenet example:

cx predict classifier samples.json debug

$ cx logs classifier  -v | grep api
...
[imagenet----classifier-77cfc96d77-qngjb api] INFO:cortex:model_signature: {'images': {'shape': [-1, -1, -1, 3], 'type': 'float32'}}
[imagenet----classifier-77cfc96d77-qngjb api] INFO:cortex:Serving on http://0.0.0.0:8888
[imagenet----classifier-77cfc96d77-qngjb api] INFO:cortex:Serving on http://[::]:8888
[imagenet----classifier-77cfc96d77-qngjb api] INFO:cortex:sample: {'url': 'https://bowwowinsurance.com.au/wp-content/uploads/2018/10/akita-700x700.'}
[imagenet----classifier-77cfc96d77-qngjb api] INFO:cortex:pre_inference: {'images': [[[[1. 1. 1.]   [1. 1. 1.]   [1. 1. 1.]   ...   [1. 1. 1.]   [1. 1. 1.] ...]}
[imagenet----classifier-77cfc96d77-qngjb api] INFO:cortex:inference: {'response': {'classes': [-0.3301149606704712, -0.3253870904445648, -0.06579694151878357, -0.1973...]}}
[imagenet----classifier-77cfc96d77-qngjb api] INFO:cortex:post_inference: {'class': 'Eskimo dog'}
[imagenet----classifier-77cfc96d77-qngjb api] INFO:cortex:sample: {'base64': '/9j/4AAQSkZJRgABAQEBLAEsAAD/4gxYSUNDX1BST0ZJTEUAAQEAAAxITGlubwIQAABtbnRy...'}
[imagenet----classifier-77cfc96d77-qngjb api] INFO:cortex:pre_inference: {'images': [[[[1. 1. 1.]   [1. 1. 1.]   [1. 1. 1.]   ...   [1. 1. 1.]   [1. 1. 1.] ...]}
[imagenet----classifier-77cfc96d77-qngjb api] INFO:cortex:inference: {'response': {'classes': [-0.3783876299858093, -0.30833104252815247, -0.1648027002811432, -0.1170...]}}
[imagenet----classifier-77cfc96d77-qngjb api] INFO:cortex:post_inference: {'class': 'Granny Smith'}
[imagenet----classifier-77cfc96d77-qngjb api] INFO:cortex:[2019-08-28T16:11:46.099194Z] 200 OK

---

Checklist:

• Run make test and make lint
• Test end to end manually (e.g. build/push all images, restart operator, and run cx refresh)
• Update examples
• Update documentation (add any new files to summary.md)
• Merge to master
• Cherry-pick into release branches if it's a bugfix
• Check gitbook that docs look good and links function properly
• Alert team if dev environment changed
• Delete the branch once it's merged