spec: add RestartPolicy type in pod manifest.

[yifan-gu]

The optional pod level RestartPolicy type defines the behavior of
the apps within a pod when they exit.

This PR adds 3 valid policies:

• always: The app always restarts when it exits, regardless of its
  exit status.
• onFailure: The app restarts only when it exits unsuccessfully.
• never: The app never restarts in any cases.

[yifan-gu]

After some discussion with @jonboulle @philips , we thought it could be less confusion if we just import the kubernetes' restart policy here.
/cc @thockin @vbatts @mpasternacki

[yifan-gu]

Not specifying the exponential delay details here as I think it's rather an implementation detail than a spec. But maybe we can leave an example here?

[yifan-gu]

ping ?

[yifan-gu]

cc @alban @iaguis

[yifan-gu]

I found a problem here is, how do we define restart? Is that just the restart of the process? how about the side effects left by the previous run of the process? Should we clean it? How?

@jonboulle @philips

[krobertson]

Think it would be good to expand the specification some to cover some of the expected behavior, such as the Kubernetes docs do. Primarily around restart policy being a pod setting and how to handle failures of individual apps. Also perhaps around expectations when restarting, such as clearing changed filesytem, and whether empty volumes are cleared.