Add healthcheck option 'request_interval' for Route53 provider

Route53 allows to specify an interval for its health checks.
To maintain backward compatibility, the default for this option
when ommited is 10 (fast check).

docs/records.md

@@ -106,6 +106,7 @@ test:
 | Key  | Description | Default |
 |--|--|--|
 | measure_latency | Show latency in AWS console | true |
+| request_interval | Healthcheck interval [10\|30] seconds | 10 |
 
 ```yaml
 ---
@@ -118,6 +119,7 @@ test:
     route53:
       healthcheck:
         measure_latency: false
+        request_interval: 30
 ```