Provides an interface for other plugins to support rebuild plugin.

When a new plugin with its own parameter value comes, we need to add a new jelly file for that plugin to rebuild plugin (as in bf501fb).
This means rebuild plugin must know all Jenkins plugins, which is not realistic.

This pull request add a feature for other plugins to support rebuild plugin.
If a plugin with its own parameter wants to work with rebuild plugin, it does as followings:
* Have its parameter value class to implement `RebuildableParameterValue`.
* Define `getRebuildPage()`, which returns a hint to access a jelly file for that parameter value.

If the parameter value class does not support `RebuildableParameterValue`, rebuild plugin works as now.