bugfix ebs volume tag behaviour
bugfix ebs volume tag behaviour

This commit modifies the response format of the ec2 calls
`create_volume` and `describe_volumes`. Previously, these calls would
always include a `Tags` key in the response, even when a volume has no tags.
Now, the `Tags` key will not be included in the response if the volume
has no tags.

When an EBS volume has no tags, calls to the aws ec2 endpoints `create_volume`
and `describe_volumes` do not include the `Tags` key in the
`response.Volumes[]` object.

However, moto does include the `Tags` key in this case. This discrepancy
in behaviour can result in code passing a moto test but failing in
production.

Sample snippets that trigger this condition:

```
def create_volume_and_then_get_tags_from_response():
    client = boto3.client('ec2', region_name='us-east-1')
    volume_response = client.create_volume(
        Size=10,
        AvailabilityZone='us-east-1a'
    )
    keys = volume_response['Keys']
```

```
def create_volume_and_then_get_tags_from_describe_volumes():
    client = boto3.client('ec2', region_name='us-east-1')
    volume_response = client.create_volume(
        Size=10,
        AvailabilityZone='us-east-1a'
    )
    volume_describe_response = client.describe_volumes()
    keys = volume_describe_response['Volumes'][0]['Keys']
```

Both sample snippets will succeed in a moto test, but fail with a
`KeyError` when using the aws api.

whummer/moto,spulec/moto,whummer/moto,spulec/moto,whummer/moto,william-richard/moto,whummer/moto,spulec/moto,spulec/moto,spulec/moto,william-richard/moto,whummer/moto,spulec/moto,william-richard/moto,william-richard/moto,william-richard/moto,whummer/moto,william-richard/moto