Remove invalid RestartPolicy for cert-deployer about rke

[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.