Support Work Queue jobs with variable parallelism

When job.spec.completions is nil, only
one task needs to succeed for the job to succeed,
and parallelism can be scaled freely during runtime.

Added tests.

Release Note:

This causes two minor changes to the API.

First, unset parallelism previously was defaulted to be
equal to completions.  Now it always defaults to 1 if unset.

Second, having parallelism=N and completions unset would previously
be defaulted to 1 completion and N parallelism.
(this is not something we expect people to do, though)
Now, no defaulting occurs in that case, and the job's
behavior is different (any completion causes success).

examples/job/work-queue-2/README.md

@@ -185,6 +185,8 @@ using above links. Then build the image:
 $ docker build -t job-wq-2 .
 ```
 
+### Push the image
+
 For the [Docker Hub](https://hub.docker.com/), tag your app image with
 your username and push to the Hub with the below commands. Replace
 `<username>` with your Hub username.
@@ -194,6 +196,9 @@ docker tag job-wq-2 <username>/job-wq-2
 docker push <username>/job-wq-2
 ```
 
+You need to push to a public repository or [configure your cluster to be able to access
+your private repository](../../../docs/user-guide/images.md).
+
 If you are using [Google Container
 Registry](https://cloud.google.com/tools/container-registry/), tag
 your app image with your project ID, and push to GCR. Replace
@@ -220,7 +225,6 @@ spec:
   selector:
     matchLabels:
       app: job-wq-2
-  completions: 1
   parallelism: 2
   template:
     metadata:
@@ -263,17 +267,19 @@ Now wait a bit, then check on the job.
 $ ./kubectl describe jobs/job-wq-2 
 Name:		job-wq-2
 Namespace:	default
-Image(s):	gcr.io/causal-jigsaw-637/job-wq-2
+Image(s):	gcr.io/exampleproject/job-wq-2
 Selector:	app in (job-wq-2)
 Parallelism:	2
-Completions:	1
+Completions:	Unset
+Start Time:	Mon, 11 Jan 2016 17:07:59 -0800
 Labels:		app=job-wq-2
-Pods Statuses:	0 Running / 1 Succeeded / 0 Failed
+Pods Statuses:	1 Running / 0 Succeeded / 0 Failed
 No volumes.
 Events:
-  FirstSeen	LastSeen	Count	From	SubobjectPath	Reason			Message
-  ─────────	────────	─────	────	─────────────	──────			───────
-  1m		1m		1	{job }			SuccessfulCreate	Created pod: job-wq-2-7r7b2
+  FirstSeen	LastSeen	Count	From			SubobjectPath	Type		Reason			Message
+  ---------	--------	-----	----			-------------	--------	------			-------
+  33s		33s		1	{job-controller }			Normal		SuccessfulCreate	Created pod: job-wq-2-lglf8
+
 
 $ kubectl logs pods/job-wq-2-7r7b2
 Worker with sessionID: bbd72d0a-9e5c-4dd6-abf6-416cc267991f

examples/job/work-queue-2/job.yaml

@@ -6,7 +6,6 @@ spec:
   selector:
     matchLabels:
       app: job-wq-2
-  completions: 1
   parallelism: 2
   template:
     metadata: