Allow customizing ExecStart command

[mcg]

JVM usage varies widely. Need to be able to customize JVM arguments passed into the systemd service templet for the ExecStart.

[Matt-Yorkley]

Sounds useful. You want to submit a PR? Maybe something like this in the service:

{{ metabase_jvm_args | default(omit) }}

[mcg]

I'll cut a PR for this.

Using {{ metabase_jvm_args | default(omit) }} we won't have a default set in defaults/main.yml for this. Is that an issue and if so, how do you want that exposed to the user, readme or?