Don't create volumes for AMIs (#1456)

* Delete the volume used during AMI creation

Creating an AMI doesn't actually result in the creation of an EBS
volume, although the associated snapshot does reference one. To that
end, delete the volume once we've used it.

* Add `owner_id` to `Snapshot`, verify AMI snapshots

The default AMIs which are created by moto have EBS volume mappings
but the snapshots associated with those don't have the correct
owners set.

This adds the owner to the snapshot model and passes it through from
the JSON data.

moto/ec2/responses/elastic_block_store.py

@@ -229,7 +229,7 @@ CREATE_SNAPSHOT_RESPONSE = """<CreateSnapshotResponse xmlns="http://ec2.amazonaw
   <status>pending</status>
   <startTime>{{ snapshot.start_time}}</startTime>
   <progress>60%</progress>
-  <ownerId>123456789012</ownerId>
+  <ownerId>{{ snapshot.owner_id }}</ownerId>
   <volumeSize>{{ snapshot.volume.size }}</volumeSize>
   <description>{{ snapshot.description }}</description>
   <encrypted>{{ snapshot.encrypted }}</encrypted>
@@ -245,7 +245,7 @@ DESCRIBE_SNAPSHOTS_RESPONSE = """<DescribeSnapshotsResponse xmlns="http://ec2.am
              <status>{{ snapshot.status }}</status>
              <startTime>{{ snapshot.start_time}}</startTime>
              <progress>100%</progress>
-             <ownerId>123456789012</ownerId>
+             <ownerId>{{ snapshot.owner_id }}</ownerId>
             <volumeSize>{{ snapshot.volume.size }}</volumeSize>
              <description>{{ snapshot.description }}</description>
              <encrypted>{{ snapshot.encrypted }}</encrypted>