9353: Add libcloud.common.BaseHTTPError to CLOUD_ERRORS.
Due to eventual consistency, it seems that calling ex_create_tags() on a node
that has just been created can return "InvalidInstanceID.NotFound" for some
period. The ec2 driver raises BaseHTTPError when it doesn't recognize a more
specific error, however prior to this commit, this wasn't recognized as a
retryable error, so node manager would fall over. This commit makes
BaseHTTPError a retryable error.