Shorten capture_name_prefix

While including "stemcell" and the OS and version makes it obvious what
the final VHD file is, there is a 23 character limit for the
capture_name_prefix in Packer. The storage account +
capture_container_name should already make it obvious that it is a
stemcell, and is of a particular OS, so only include the version as the
prefix.

Author: ystros

lib/packer/config/azure.rb

@@ -33,7 +33,7 @@ def builders
                 'temp_resource_group_name' => "#{@vm_prefix}-#{Time.now.to_i}",
                 'storage_account' => @storage_account,
                 'capture_container_name' => @os,
-                'capture_name_prefix' => "bosh-stemcell-#{@os}-#{@version}",
+                'capture_name_prefix' => "#{@version}",
                 'image_publisher' => 'MicrosoftWindowsServer',
                 'image_offer' => ENV['BASE_IMAGE_OFFER'],
                 'image_sku' => ENV['BASE_IMAGE'],

spec/packer/config/azure_spec.rb

@@ -40,7 +40,7 @@
         'temp_resource_group_name' => "some-vm-prefix-#{Time.now.to_i}",
         'storage_account' => 'some-storage-account',
         'capture_container_name' => 'test-os',
-        'capture_name_prefix' => 'bosh-stemcell-test-os-2019.9999',
+        'capture_name_prefix' => '2019.9999',
         'image_publisher' => 'MicrosoftWindowsServer',
         'image_offer' => 'some-base-image-offer',
         'image_sku' => 'some-base-image',