Add connection-type to provider diskimage

The connection type should be included in the provider diskimage. This
makes it possible to define images using other connection methods than
ssh like winrm for Windows.

Change-Id: Ica0b9afe39d347028eb66c069b8dbd56a8c0ec8c

nodepool/tests/fixtures/config_validate/good.yaml

@@ -54,11 +54,13 @@ providers:
     diskimages:
       - name: trusty
         pause: False
+        connection-type: ssh
     cloud-images:
       - name: trusty-unmanaged
         config-drive: true
       - name: windows-unmanaged
         username: winzuul
+        connection-type: winrm
     pools:
       - name: main
         max-servers: 184