test_versioning_concurrent_multi_object_delete creates the bucket twice and fails on BucketAlreadyExists

Hey!
While running test_versioning_concurrent_multi_object_delete on NooBaa, I saw that the test is trying to create the bucket twice -
1. on `get_new_bucket()` - `client.create_bucket(Bucket=name)`
2. on `_create_objects()` -> `get_new_bucket_resource()` -> `bucket.create()`

See code references after the suggested fix.

According to my understanding, these are the needed changes for fixing the issue -
```diff
def test_versioning_concurrent_multi_object_delete():
num_objects = 5
num_threads = 5
- bucket_name = get_new_bucket()
+ bucket = get_new_bucket_resource()
+ bucket_name = bucket.name

check_configure_versioning_retry(bucket_name, "Enabled", "Enabled")

key_names = ["key_{:d}".format(x) for x in range(num_objects)]
- bucket = _create_objects(bucket_name=bucket_name, keys=key_names)
+ _create_objects(bucket=bucket, bucket_name=bucket_name, keys=key_names)

client = get_client()
versions = client.list_object_versions(Bucket=bucket_name)['Versions']
```

It would be great if you would fix that, thanks a lot!

**Code References -**
1. get_new_bucket() -
```python
def get_new_bucket(client=None, name=None):
"""
Get a bucket that exists and is empty.

Always recreates a bucket from scratch. This is useful to also
reset ACLs and such.
"""
if client is None:
client = get_client()
if name is None:
name = get_new_bucket_name()

client.create_bucket(Bucket=name) // create bucket first time
return name
```

2. get_new_bucket_resource() -
```python
def get_new_bucket_resource(name=None):
"""
Get a bucket that exists and is empty.

Always recreates a bucket from scratch. This is useful to also
reset ACLs and such.
"""
s3 = boto3.resource('s3',
aws_access_key_id=config.main_access_key,
aws_secret_access_key=config.main_secret_key,
endpoint_url=config.default_endpoint,
use_ssl=config.default_is_secure,
verify=config.default_ssl_verify)
if name is None:
name = get_new_bucket_name()
bucket = s3.Bucket(name)
bucket_location = bucket.create() // create bucket second time
return bucket
```

3. Test code -
```python
def test_versioning_concurrent_multi_object_delete():
num_objects = 5
num_threads = 5
bucket_name = get_new_bucket() // 1

check_configure_versioning_retry(bucket_name, "Enabled", "Enabled")

key_names = ["key_{:d}".format(x) for x in range(num_objects)]
bucket = _create_objects(bucket_name=bucket_name, keys=key_names) // 2

client = get_client()
versions = client.list_object_versions(Bucket=bucket_name)['Versions']
assert len(versions) == num_objects
objs_dict = {'Objects': [dict((k, v[k]) for k in ["Key", "VersionId"]) for v in versions]}
results = [None] * num_threads

def do_request(n):
results[n] = client.delete_objects(Bucket=bucket_name, Delete=objs_dict)

t = []
for i in range(num_threads):
thr = threading.Thread(target = do_request, args=[i])
thr.start()
t.append(thr)
_do_wait_completion(t)

for response in results:
assert len(response['Deleted']) == num_objects
assert 'Errors' not in response

response = client.list_objects(Bucket=bucket_name)
assert 'Contents' not in response
```