Deploy simple Linux VM template should support Ubuntu 18.04 LTS

[101-vm-simple-linux](https://github.com/Azure/azure-quickstart-templates/tree/master/101-vm-simple-linux/)

### Issue Details
In the parameters section of azure-quickstart-templates/101-vm-simple-linux/azuredeploy.json, there is no allowed value for Ubuntu 18.04 LTS which is now the latest Ubuntu LTS version:

"ubuntuOSVersion": {
"type": "string",
"defaultValue": "16.04.0-LTS",
"allowedValues": [
"12.04.5-LTS",
"14.04.5-LTS",
"15.10",
"16.04.0-LTS"
],
"metadata": {
"description": "The Ubuntu version for the VM. This will pick a fully patched image of this given Ubuntu version."
}

Open 101-vm-simple-linux/azuredeploy.json and inspect the ubuntuOSVersion parameter's defaultValue and allowedValues. Confirm the template accepts Ubuntu 18.04 LTS and that the parameter description remains accurate.

Written by the indexing model from the issue text.