Fixes for argument handling in AshScriptPlugin's script template

[kevinmehall]

• Previously the -main argument was ignored because the custom_mainclass variable was set but never used. process_args is called after app_mainclass is set to the default value, so this PR makes it simply overwrite app_mainclass instead of introducing another variable.
• Previously the additional arguments passed on the command line were passed to java before the defaults, which means the defaults take precedence and couldn't be overridden. This puts the flags from JAVA_OPTS and the command line after the defaults, which matches the behavior of the Bash template.

[lightbend-cla-validator]

Hi @kevinmehall,

Thank you for your contribution! We really value the time you've taken to put this together.

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[lightbend-cla-validator]

Hi @kevinmehall,

Thank you for your contribution! We really value the time you've taken to put this together.

Before we proceed with reviewing this pull request, please sign the Lightbend Contributors License Agreement:

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[muuki88]

Can you sign the Contributor CLA?

[lightbend-cla-validator]

Hi @kevinmehall,

Thank you for your contribution! We really value the time you've taken to put this together.

Before we proceed with reviewing this pull request, please sign the Lightbend Contributors License Agreement:

https://www.lightbend.com/contribute/cla