qa/rgw/s3tests: remove 'client.0' from bucket prefix

qa/rgw/s3tests: remove 'client.0' from bucket prefix

new sns test cases are using this for topic names, but the '.' is not
allowed there:

> api_params = {'Name': 'test-client.0-n3bdgre5el2jk8v-606'}
> botocore.exceptions.ClientError: An error occurred (InvalidArgument) when calling the CreateTopic operation: Name must be made up of only uppercase and lowercase ASCII letters, numbers, underscores, and hyphens

Signed-off-by: Casey Bodley <cbodley@redhat.com>