configurable retry policy for system.log.process and system.metrics

The retry policies for system.log.process.retry.policy.max.delay.ms and system.metrics.retry.policy.max.delay.ms are defaulted to fixed.delay and there are concerns about running with other policy for types (exponential.backoff). The properties were left out of cdap-default.xml.

This leads to awkward usage. We'll need to add these properties into the next release in the cdap-default.xml, Ambari service and CSD.