out_opensearch: fix default value of refresh_credentials_interval

[Watson1978]

Fix #130

Currently, when using the default value for refresh_credentials_interval,
the string "5h" is passed to the interval argument of timer_execute method.

However, interval argument supports integer or float value.

Ref. https://docs.fluentd.org/plugin-helper-overview/api-plugin-helper-timer#timer_execute-title-interval-repeat-true-and-block

[daipom]

Thanks!

The type of refresh_credentials_interval is time, but the default value should be the raw value.