Fixed ignored RDS backup_retention_period when value was 0

Commit https://github.com/juanfont/boto/commit/35ee6515094e565e350409bdc7910d946564f31a fixes bug https://github.com/boto/boto/issues/1886. This issue only happened when value was 0, not when it was '0'.

It also modifies backup_retention_period type to int, as in AWS doc. Tests updated.