Plugins are unable to obtain the VelocityEngine used by Struts

Issue Summary

In Confluence 7, plugins could obtain the VelocityEngine used by Confluence and WebWork internally using `VelocityUtils#getVelocityEngine`.

In Confluence 8, the VelocityEngine used internally by Struts is separate to the one used by Confluence and can only be obtained via the Dispatcher bean which is not available to plugins.

Steps to Reproduce

1. Call VelocityUtils#getVelocityEngine from within a plugin
2. Make changes to the engine that would affect rendering of actions

Expected Results

Action rendering changes as expected

Actual Results

No change

Workaround

Obtain the Dispatcher bean via the ContainerManager after which the Struts internal VelocityManager and VelocityEngine can be obtained.