Allow customization of default PluginManager

Provide a mechanism to incorporate a custom PluginManager when launching Jenkins as a webapp.

The motivating use case for this change is custom wars that want to implement some kind of custom "bundling-like" functionality, in the sense of modifying the available plugins before they are loaded.
Because of the point in the initialization sequence the hook must be introduced, an extension point to be implemented by a plugin is not a feasible solution.