cloud: add image variant for Azure
    
    Extend cloud base image and customize for Azure:
    
    1. Don't install QEMU guest agent
    
    2. Install WALinuxAgent
    
    3. Configure chrony to use virtual PHC
    
    4. Configure sshd with ClientAliveTimeout 120 to prevent
       ssh timeouts
    
    Performed some basic tests by generating an image with:
    
    1. Using QEMU (using direct kernel boot) with
       Fedora-Server-netinst-x86_64-36-1.5.iso and a flattened
       kickstart with poweroff instead of reboot
    
    2. Converted QCOW2 to VHD using:
       qemu-img convert -o subformat=fixed,force_size -O vpc ...
    
    3. Uploading image and created VM, verifying:
    
      - cloud-init configures system
    
      - chrony using PTP configuration
    
      - waagent service installed (but not running until presets
        are approved)
    
      - SSH configuration and functional
    
    Signed-off-by: Chris Patterson <cpatterson@microsoft.com>