Feature: Add variable to configure max retention parameter.

Reason: Usually size-based deletion of the journal is sufficient with
such options as `journald_max_disk_size` and `journald_max_files`, but in
some cases time-based deletion is required in order to comply with data
retention policies.

Result: Users can configure journald MaxRetention.

Signed-off-by: Rich Megginson <rmeggins@redhat.com>