all: improved monitoring of buildlet pods after creation

* Replaced cancel with context.Context
* StartPod can be canceled
* Wait for buildlet to come online, but fail fast if pod fails first
* Support timeout waiting for pod to leave pending phase
* Use Kubernetes watch API (long poll)

Updates golang/go#12546

Change-Id: I792a3b8fed615362a0290feee7de0c2cefe43c0e
Reviewed-on: https://go-review.googlesource.com/15285
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
7 files changed