Review of patch for KNOX-179: Simple way to introduce new provider/servlet filters into the chains?

[Kevin Minder <ke...@hortonworks.com>]

Anyone have any feedback on the patch for KNOX-179?
https://issues.apache.org/jira/browse/KNOX-179
I'd like to base some work on KNOX-177 of this so I'd like to get it in.
The reason I'm reaching out is that this change opens the door for using 
class names in configuration files (ie topology files) which has always 
rubbed me the wrong way.
I've convinced myself that this is acceptable here because these are 
likely to be used for one off, in the field, customer integrations.

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