optional nodename for deployment affintiy

[salehsedghpour]

This PR closes #26

Previously we had two deployment models, one with affinity and one without affinity. Now we have one deployment model, and if the user does not enter the nodename in service description, we won't add an empty string in the generated files.

Example:
If the input json file is like this:

.
.
"services": [
    {
      "name": "service-1",
      "clusters": [
        {
          "cluster": "cluster-1",
          "namespace": "ns-1",
        }
      ],
      "resources": {
.
.
.

Before this PR the output would be like:

spec:
            nodeName: 
            containers:
                - name: app
                  image: app-demo:latest
                  imagePullPolicy: Never

After this PR the output is:

spec:
            containers:
                - name: app
                  image: app-demo:latest
                  imagePullPolicy: Never

[alekodu]

Good the code was simplified.