Fix 'add a new machine' flow for che7-workspace

[olexii4]

Description

Fix 'add a new machine' flow for che7-workspace.

Reproduction Steps

1. Create the workspace with che-kube:

{
  "defaultEnv": "default",
  "environments": {
    "default": {
      "machines": {
        "ws/dev": {
          "attributes": {
            "memoryLimitBytes": "1073741824"
          },
          "servers": {},
          "volumes": {},
          "installers": [],
          "env": {}
        }
      },
      "recipe": {
        "type": "kubernetes",
        "content": "kind: List\nitems:\n - \n  apiVersion: v1\n  kind: Pod\n  metadata:\n   name: ws\n  spec:\n   containers:\n    - \n     image: 'rhche/centos_jdk8:latest'\n     name: dev\n     resources:\n      limits:\n       memory: 1024Mi\n",
        "contentType": "application/x-yaml"
      }
    }
  },
  "commands": [],
  "projects": [],
  "name": "che7-workspace",
  "attributes": {
    "editor": "org.eclipse.che.editor.theia:1.0.0",
    "plugins": "che-machine-exec-plugin:0.0.1"
  },
  "links": []
}

2. Try to add a new machine.

Expected behavior:
Add a new machine into the workspace.

Observed behavior:
An error "Che plugin configuration can be applied to a workspace with one pod only"

OS and version:
Che 6.15.0; Chrome 63.0; Ubuntu 18.10
Diagnostics: