Allow to use JVM default truststore

[notizklotz]

It seems that with the default splunk.hec.ssl.validate.certs=true one must configure a custom truststore path and it's not possible to use the JVM default truststore.

I think the config validation here is too strict: https://github.com/splunk/kafka-connect-splunk/blob/release-2.2.2/src/main/java/com/splunk/kafka/connect/SplunkSinkConnectorConfig.java#L540

The code actually creating the httpClient would handle the case alright without modifications: https://github.com/splunk/kafka-connect-splunk/blob/release-2.2.2/src/main/java/com/splunk/hecclient/Hec.java#L279

[notizklotz]

Duplicate of #441.