Set hudson-plugin-parent as parent pom.

We are going to support hudson in this fork so we use parent.pom from
Eclipse foundation.
Fix configuration page and form fields validation.

Configuration page has been broken because Form.findMatchingInput
function didn't work as expected. It was decided not to investigate
this issue but simply replace this function use with
'this.form.elements[required input].value'. Also at server side we
should decode values in order to support whitespaces in parameters
values.
Do a small refactoring and code formating.

Code is simplified and formated in order to simplify local
maintenance and future features implementation. We are not going to
create pull requests to the original repository so we can easily do
such things.