[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.

Comienza localizando el enum KnownLinuxVirtualMachineImage y compara sus entradas con las versiones de imagen de Azure que deberían ser compatibles. Revisa WithLinuxRootUsernameManagedOrUnmanaged conLatestLinuxImage(String publisher, String offer, String sku) como contexto. Se considera terminado al añadir al menos Ubuntu 18.04 y mantener actualizadas las entradas no obsoletas del enum.

Escrito por el modelo de indexación a partir del texto del issue.