Make ip_addr optional in Route53 HealthCheck

According to [AWS documentation](https://docs.aws.amazon.com/Route53/latest/APIReference/API_CreateHealthCheck.html), `IPAddress` in `HealthCheck` is optional.

This pull request makes using `fqdn` rather than `ip_addr` as health check target possible. For now users can pass `ip_addr=None` in order to bypass that param and use `fqdn`.

To make `ip_addr` truly optional, I have to change the `HealthCheck` constructor from
```
def __init__(self, ip_addr, port, hc_type, resource_path, fqdn=None, string_match=None, request_interval=30, failure_threshold=3):
```
to
```
def __init__(self, port, hc_type, resource_path, ip_addr=None, fqdn=None, string_match=None, request_interval=30, failure_threshold=3):
```
But that may break a lot of users code. Probably that can happen in next major version update (3.0?).