fix sanity checks to make shippable happy (ansible#39685)

Signed-off-by: Adam Miller <admiller@redhat.com>

lib/ansible/modules/cloud/openstack/os_volume_snapshot.py

@@ -43,7 +43,7 @@
      description:
        - String describing the snapshot
      required: false
-     default: None
+     default: null
      aliases: ['description']
    volume:
      description:
@@ -55,6 +55,7 @@
          is attached to an instance.
      required: false
      default: False
+     type: bool
    state:
      description:
        - Should the resource be present or absent.