feat: server variables must have default value (#322)

This PR brings the capability of checking whether
a server variable has a default value or not. If not,
then we are going to display warning.

The code snippet below will display warning.
```
servers:
  - url: 'https://example.com'
    description: 'A server which has variable without default value'
    variables:
      name:
        default: ''
```

.secrets.baseline

@@ -3,7 +3,7 @@
     "files": "package-lock.json|^.secrets.baseline$",
     "lines": null
   },
-  "generated_at": "2021-02-24T19:25:15Z",
+  "generated_at": "2021-09-15T12:11:12Z",
   "plugins_used": [
     {
       "name": "AWSKeyDetector"
@@ -25,6 +25,7 @@
       "name": "CloudantDetector"
     },
     {
+      "ghe_instance": "github.ibm.com",
       "name": "GheDetector"
     },
     {
@@ -85,7 +86,7 @@
       }
     ]
   },
-  "version": "0.13.1+ibm.29.dss",
+  "version": "0.13.1+ibm.45.dss",
   "word_list": {
     "file": null,
     "hash": null