admin: add restart as an option.
admin: add restart as an option.

- Due to server caching, sometimes it may be necessary to restart the
  server to flush the cache*. Or for other reasons (out of log space,
  etc).
- Added documentation on GCE Admin doc.

Tested manually a few times on test servers and frontend.
shellcheck (lint for shell) error-free.

* There are other ways to flush the cache such as having a /flush
  handler, which we will need at some point, but not yet. In fact, the
  handler will be called /notify, to receive GCS change notifications on
  the buckets but it will be a fine-grained flush, just removing the
  items updated. But I digress.

Change-Id: I0941fa485425e56684c34b651ff5dde993f52ae4
Reviewed-on: https://upspin-review.git.corp.google.com/1671
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

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