Remove invalid RestartPolicy for cert-deployer

[superseb]

We specify never as RestartPolicy for the cert-deployer image (https://github.com/rancher/rke/blob/master/pki/deploy.go#L79) but this is not a valid value. The reason it works is that it's not verified in the API version used in Docker 1.12 (1.24). We should not specify the RestartPolicy, as default is "" which is do no restart.

Next to being invalid, this also blocks using newer Docker versions as validation to RestartPolicy was added in API version 1.25.

[galal-hussein]

Reopening for QA to test with newer docker versions

[soumyalj]

Reproduced the issue with rke version v0.0.7-dev using Docker 17.03.2-ce version hosts.
Kubernetes cluster creation failed with the following error :

rke soumya$ ./rke_darwin-amd64-0.0.7-dev up --config cluster.yml 
INFO[0000] Building Kubernetes cluster                  
INFO[0000] [ssh] Checking private key                   
INFO[0000] [ssh] Start tunnel for host [45.55.72.189]   
INFO[0000] [ssh] Start tunnel for host [45.55.72.189]   
INFO[0000] [ssh] Start tunnel for host [45.55.74.191]   
INFO[0000] [certificates] Generating kubernetes certificates 
INFO[0000] [certificates] Generating CA kubernetes certificates 
INFO[0000] [certificates] Generating Kubernetes API server certificates 
INFO[0000] [certificates] Generating Kube Controller certificates 
INFO[0000] [certificates] Generating Kube Scheduler certificates 
INFO[0000] [certificates] Generating Kube Proxy certificates 
INFO[0001] [certificates] Generating Node certificate   
INFO[0001] [certificates] Generating admin certificates and kubeconfig 
INFO[0001] [certificates] Deploying kubernetes certificates to Cluster nodes 
FATA[0004] Failed to create Certificates deployer container on host [45.55.72.189]: Error response from daemon: invalid restart policy 'never' 

Tested with rke version v0.0.8-dev using docker 17.03.2-ce and docker 1.13.1 hosts. Cluster creation was successful. No errors were seen. Pod creation was successful.