Cache flush on VDB installation does not work

Problem/Motivation

After you install a VDB provider that exposes a VDBProviderInterface plugin, the plugin manager does not flush the cache for the plugins.

This causes you to have to flush the caches manually, before the VDB provider is usable.

Steps to reproduce (required for bugs, but not feature requests)

* Install a VDB provider
* Try to get the definition for it, before flushing the cache.

Proposed resolution

* Make sure the cache is updated on installation.