add UpdateFunc to handle update events

[mqliang]

• Add an Update function to the controller.
• The informer periodically generates an Update event but we aren't processing these because we don't have an update function.
• As a result, TrainingJob.reconcile doesn't get called periodically and we aren't properly updating the status of the job.

ref #309
ref #293

This change is 

[coveralls]

Coverage increased (+0.05%) to 31.664% when pulling 2383700 on mqliang:resync into 98a34a1 on tensorflow:master.

[jlewi]

Thanks for this PR; unfortunately this is blocked on fixing head #293.

In the meantime could you update the PR description to explain in more detail what this PR is addressing?

[jlewi]

LGTM. I'm going to merge this PR so I can merge it into #308.
I'll fix the lint error in that PR; doesn't look like this PR is what caused the lint error which is what's causing travis build to fail.

[gaocegege]

I am not sure if updatefunc for TFJob works, since I think updatefuncs for job and service are also necessary to update the TFJob status 🤔

WDYT @mqliang

[mqliang]

@gaocegege
Yes, you are right. We should also watch events for jobs/services and process their corresponding TFJobs.

I will send a PR to address this.