[FEATURE REQ] Keep enum KnownLinuxVirtualMachineImage Updated

It would be good to keep up to date enum KnownLinuxVirtualMachineImage

Currently it contains only these non-deprecated fields:
```
/** UbuntuServer 14.04LTS. */
UBUNTU_SERVER_14_04_LTS("Canonical", "UbuntuServer", "14.04.4-LTS"),
/** UbuntuServer 16.04LTS. */
UBUNTU_SERVER_16_04_LTS("Canonical", "UbuntuServer", "16.04.0-LTS"),
/** Debian 8. */
DEBIAN_8("credativ", "Debian", "8"),
/** CentOS 7.2. */
CENTOS_7_2("OpenLogic", "CentOS", "7.2"),

```

It would be good to have at least Ubuntu 18.04

I understand that you prodive the method `WithLinuxRootUsernameManagedOrUnmanaged withLatestLinuxImage(String publisher, String offer, String sku);`, but keeping KnownLinuxVirtualMachineImage up to date is much more convenient. Keeping it up to date also seems quite easy for developers.

まず KnownLinuxVirtualMachineImage enum を見つけ、そのエントリをサポート対象にすべき Azure イメージのバージョンと比較します。コンテキストとして、WithLinuxRootUsernameManagedOrUnmanaged の withLatestLinuxImage(String publisher, String offer, String sku) を確認してください。少なくとも Ubuntu 18.04 を追加し、enum の非推奨ではないエントリを最新に保つことが完了の条件です。

索引モデルが issue の本文から書いたものです。