intro tutorial updates

[amygdala]

tracking issue: #1221 .
Accompanying update to the example code: kubeflow/examples#658 (pin the TF version in the web-ui dockerfile) .

My text editor trims EOL whitespace, so there are a bunch of diffs for those. The changes of substance:

• changed the tf-serving service from mnist-gcs-dist to mnist-service so that the web UI would work out of the box. (If there is a good reason for using the other name, we can have them edit the service in the web ui, but they'll see that scary error first)
• when they set up their bucket paths, add a timestamp-based subdir (so that they can run the example more than once)
• since the web-ui service is using ClusterIP, and not generating an external IP, changed the instructions to have them port-forward. (I left some of the old stuff in but commented out -- feel free to ask me to just delete)
• fixed some instructions on how to edit the gcr image in the web ui deployment.yaml file.

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This change is 

[amygdala]

/assign @sarahmaddox , + could you take a look as well, Dan? @dansanche

[sarahmaddox]

Thanks Amy! Just a few comments from a tech writing point of view.

[sarahmaddox]

/assign

[amygdala]

PTAL again.

[sarahmaddox]

Thanks @amygdala This looks great.
/lgtm
/approve

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