Check bucket name length at CreateBucket

Check that s3 bucket names follow the documented length restriction:

'Bucket names must be at least 3 and no more than 63 characters long.'

See https://docs.aws.amazon.com/AmazonS3/latest/dev/BucketRestrictions.html

moto/s3/exceptions.py

@@ -178,3 +178,13 @@ class InvalidStorageClass(S3ClientError):
             "InvalidStorageClass",
             "The storage class you specified is not valid",
             *args, **kwargs)
+
+
+class InvalidBucketName(S3ClientError):
+    code = 400
+
+    def __init__(self, *args, **kwargs):
+        super(InvalidBucketName, self).__init__(
+            "InvalidBucketName",
+            "The specified bucket is not valid.",
+            *args, **kwargs)