[mgmt] test mpg sample generation

[kazrael2119]

steps:

1. run rush update && rush build -t @azure/arm-mongocluster
2. run rushx build:samples
3. run rushx execute:samples

[azure-sdk]

API change check

API changes are not detected in this pull request.

[kazrael2119]

manually change:
1.update build:samples and execute:samples scripts
2. add

"//sampleConfiguration": {
    "productName": "",
    "productSlugs": [
      "azure"
    ],
    "disableDocsMs": true,
    "apiRefLink": "https://docs.microsoft.com/javascript/api/@azure/arm-networkanalytics?view=azure-node-preview"
  },

in package.json

3. create a sample.env file with

# App registration secret for AAD authentication
AZURE_CLIENT_SECRET=
AZURE_CLIENT_ID=
AZURE_TENANT_ID=