Fix a plugins glitch.

The before_dispatch method in configuration plugins is allowed to alter
command line arguments, to add or remove arguments needed to support
specific environments. The default implementation of plugins doesn't
provide any return value for the plugin methods. For before_dispatch,
this meant that the default plugin method effectively removed all command
line parameters!

This change just turns the default before_dispatch plugin method into an
effective no-op. If the user doesn't provide a before_dispatch method,
the default implementation will leave the arguments correctly unchanged.

Bugs closed: aurora-362

Reviewed at https://reviews.apache.org/r/20707/