[azure-arm] In debug mode, information about shared_image_gallery source is missing

_This issue was originally opened by @feiyushi as hashicorp/packer#7702. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

The printout is missing the source shared_image_gallery information, as shown below. I'd love to see that in there.
Thanks.

packer version: 1.4.0
Host platform: Ubuntu 16.04
Debug log (partial):
```
2019/05/30 13:08:15 Running builder: azure-arm
2019/05/30 13:08:15 [INFO] (telemetry) Starting builder azure-arm
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 :: Configuration
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 CaptureNamePrefix=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 CaptureContainerName=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 ImagePublisher=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 ImageOffer=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 ImageSku=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 ImageVersion=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 ImageUrl=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 CustomManagedImageResourceGroupName=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 CustomManagedImageName=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 Location=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 VMSize=Standard_D1_v2
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 ManagedImageResourceGroupName=2o2kg2yipin
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 ManagedImageName=packerImg
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 ManagedImageStorageAccountType=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 ManagedImageOSDiskSnapshotName=packerImg_os_disk
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 ManagedImageDataDiskSnapshotPrefix=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 ResourceGroupName=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 StorageAccount=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 TempComputeName=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 TempResourceGroupName=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 BuildResourceGroupName=2o2kg2yipin
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 VirtualNetworkName=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 VirtualNetworkSubnetName=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 VirtualNetworkResourceGroupName=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 CustomDataFile=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 OSType=Linux
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 DiskCachingType=
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 UserName=packer
2019/05/30 13:08:15 packer: 2019/05/30 13:08:15 Password=********************************
```

Start by tracing the azure-arm debug-mode configuration output shown in the issue and find where source image fields are printed. Check how shared_image_gallery input is represented there; done means the shared_image_gallery source information appears in the debug printout when that source is configured.

Written by the indexing model from the issue text.