Cloudera Director template parameter vnetNewOrExisting not accepting "existing"

[Cloudera Director](https://github.com/Azure/azure-quickstart-templates/tree/master/cloudera-director-on-centos)

### Issue Details

When reviewing the template for Cloudera Directory on CentOS template, the vnetNewOrExisting appears to only accept a value for "new". It appears to concatenate this value to use different .json files. However, there are files with "existing" in their names. It appears as if the template was meant to have both "new" and "existing" as valid values and not just "new".

Open the linked Cloudera Director on CentOS template and inspect the vnetNewOrExisting parameter alongside the referenced .json filenames. Verify how the parameter is validated and concatenated, then test that the existing value selects the existing-network template while new continues to select the new-network template.

Written by the indexing model from the issue text.