Duplicate default JVM options

[LionelCons]

service.pp contains default JVM options in $default_jvm_options but they don't seem to be used.

Instead, defaults are hard-coded in jvm.options.erb.

Defaults should be defined only once.

[martin-koerner]

Additionally, the defaults (-Xms=256m -Xmx1g) don't match the docs: "Always make sure to set the minimum (Xms) and maximum (Xmx) heap allocation size to the same value to prevent the heap from resizing at runtime, which is a very costly process. " (https://www.elastic.co/guide/en/logstash/current/performance-troubleshooting.html)