Provide relevant error message when unsupported auth strategy and network plugins are used in cluster.yml file

[soumyalj]

1. Create a cluster with one master [controlplane, etcd] node and the other worker node using cluster.yml file. Use the command ./rke_darwin-amd64 cluster up --cluster-file cluster.yml
   Cluster.yml file example:

auth:
  strategy: x200
network:
  plugin: abcd
hosts:
  - advertised_hostname: server1
    ip: 1.1.1.1
    user: ubuntu
    role: [controlplane, etcd]
    docker_socket: /var/run/docker.sock
    advertise_address: 10.1.1.1
  - advertised_hostname: server2
    ip: 2.2.2.2
    user: ubuntu
    role: [worker]
    advertise_address: 10.2.2.2

services:
  etcd:
    image: quay.io/coreos/etcd:latest
  kube-api:
    image: quay.io/coreos/hyperkube:v1.7.5_coreos.0
    service_cluster_ip_range: 10.233.0.0/18
    extra_args:
      v: 4
  kube-controller:
    image: quay.io/coreos/hyperkube:v1.7.5_coreos.0
    cluster_cidr: 10.233.64.0/18
    service_cluster_ip_range: 10.233.0.0/18
  scheduler:
    image: quay.io/coreos/hyperkube:v1.7.5_coreos.0
  kubelet:
    image: quay.io/coreos/hyperkube:v1.7.5_coreos.0
    cluster_domain: cluster.local
    cluster_dns_server: 10.233.0.3
    infra_container_image: gcr.io/google_containers/pause-amd64:3.0
  kubeproxy:
    image: quay.io/coreos/hyperkube:v1.7.5_coreos.0

Cluster creation fails but does not provide any indication that the fields are unsupported/invalid.
It fails with the below error:

soumyas-MBP:rke soumya$ ./rke_darwin-amd64 cluster up --cluster-file clusterdigworking.yml
INFO[0000] Building Kubernetes cluster                  
INFO[0000] [ssh] Checking private key                   
INFO[0000] [ssh] Start tunnel for host [rketest1] 
INFO[0000] [ssh] Start tunnel for host [rketest1] 
INFO[0000] [ssh] Start tunnel for host [rketes2]  
INFO[0000] [state] Found local kube config file, trying to get state from cluster 
INFO[0000] [state] Fetching cluster state from Kubernetes 
INFO[0030] Timed out waiting for kubernetes cluster to get state 
INFO[0030] [etcd] Building up Etcd Plane..              
INFO[0031] [etcd] Successfully pulled etcd image on host [rketest1] 
INFO[0031] [etcd] Successfully started etcd container on host [rketest1] 
INFO[0031] [etcd] Successfully started Etcd Plane..     
INFO[0031] [controlplane] Building up Controller Plane.. 
INFO[0033] [controlplane] Successfully pulled kube-api image on host [rketest1] 
INFO[0033] [controlplane] Successfully started kube-api container on host [rketest1] 
INFO[0034] [controlplane] Successfully pulled kube-controller image on host [rketest1] 
INFO[0034] [controlplane] Successfully started kube-controller container on host [soumyarketest1] 
INFO[0034] [controlplane] Successfully pulled scheduler image on host [rketest1] 
INFO[0035] [controlplane] Successfully started scheduler container on host [rketest1] 
INFO[0035] [controlplane] Successfully started Controller Plane.. 
INFO[0035] [worker] Building up Worker Plane..          
INFO[0035] [worker] Successfully pulled kubelet image on host [rketest1] 
INFO[0035] [worker] Successfully started kubelet container on host [rketest1] 
INFO[0036] [worker] Successfully pulled kube-proxy image on host [rketest1] 
INFO[0036] [worker] Successfully started kube-proxy container on host [rketest1] 
INFO[0038] [worker] Successfully pulled nginx-proxy image on host [rketes2] 
INFO[0038] [worker] Successfully started nginx-proxy container on host [rketes2] 
INFO[0038] [worker] Successfully pulled kubelet image on host [rketes2] 
INFO[0039] [worker] Successfully started kubelet container on host [rketes2] 
INFO[0039] [worker] Successfully pulled kube-proxy image on host [rketes2] 
INFO[0040] [worker] Successfully started kube-proxy container on host [rketes2] 
INFO[0040] [worker] Successfully started Worker Plane.. 
INFO[0040] [reconcile] Reconciling cluster state        
INFO[0040] [reconcile] This is newly generated cluster  
INFO[0040] [certificates] Save kubernetes certificates as secrets 
INFO[0040] [certificates] Successfuly saved certificates as kubernetes secret [k8s-certs] 
INFO[0040] [state] Saving cluster state to Kubernetes   
FATA[0070] [state] Failed to save configuration state: [state] Timeout waiting for kubernetes to be ready 

Relevant error messages must be provided to help the user fix the issue.

[soumyalj]

Tested with rke version v0.0.6-dev. Relevant error messages are provided when unsupported strategy and plugins are used:

soumyas-MBP:rke soumya$ ./rke_darwin-amd64 up --config clusterdigworking.yml
INFO[0000] Building Kubernetes cluster                  
FATA[0000] Failed to validate cluster: Authentication strategy [x200] is not supported 

soumyas-MBP:rke soumya$ ./rke_darwin-amd64 up --config clusterdigworking.yml
INFO[0000] Building Kubernetes cluster                  
FATA[0000] Failed to validate cluster: Network plugin [abcd] is not supported