create cluster with empty node groups

[Vadim Rovachev]

step 1.
   Create cluster_template without node processes.
   by example:
      POST http://<host>:<port>/v1.0/<tenant>/cluster-templates
   body:
{
    "name": "example-template",
    "plugin_name": "vanilla",
    "hadoop_version": "1.1.2",
    "cluster_configs": {},
    "node_groups": [ ]
}

In UI
   Savanna -> Cluster Templates -> Create Template -> Create -> Template Name: example-template-name -> Create

step 2.
   Create cluster with cluster_template from step 1.
   by example:
      POST http://<host>:<port>/v1.0/<tenant>/clusters
   body:
{
    "name": "example-cluster",
    "plugin_name": "vanilla",
    "hadoop_version": "1.1.2",
    "cluster_template_id" : "<template_id_from_step_1>",
   "user_keypair_id": "vrovachev",
   "default_image_id": "<image_id>"
}

In UI
   Savanna -> Launch Cluster -> Create -> Cluster Name: example-cluster-name -> Cluster Template: example-template-name -> Create

expected result(for Example):
  HTTP 400
 cluster not launch

actual result(for Example):
 HTTP 500
 cluster launch

[Nikita Konovalov (nkonovalov)]

Creating a Cluster Template w/o Node Groups is a valid scenario since Node Groups may be specified in a Cluster Object.
API return a validation error (implemented in blueprint expose-cluster-errors-details)