External Filter Chain Definitions

This design is intended to address the following issues:

KNOX-179@jira: Simple way to introduce new servlet filters into the chains
KNOX-103@jira: Support multiple <pattern> children in <resource> for gateway.xml
KNOX-177@jira: Simplify service deployment contributor implementation

Each of these issues stem from what is currently expected of a ServiceDeploymentContributor in its contributeService method.
Basically each service deployment contributor is expected to build its own filter chain.
This is currently done by making calls to Deploymentcontext.contributeFilter.
While this provides a great deal of flexibility for each service to define a custom chain we have found that this isn't commonly necessary.
Furthermore it makes if very difficult if not impossible to introduce new filters in a chain without impacting all services.
This design will provide an abstraction to the service deployment contributors that can create either a default or specifically configured chain of filters.
The goal is to support a pattern in service deployment contributors that looks like this:

  public void contributeService( DeploymentContext context, Service service ) throws Exception {
    String chain = null; // Default if null, otherwise specific chain name defined in topoloy.xml
    Map<String,Map<String,String>> params = null; // Default if null, otherwise map of per provider role map of name/value pairs.  
    ResourceDescriptor resource = context.addResource()
    resource.role( "WEBHDFS" );
    resource.pattern( "webhdfs/v1/?**" );
    resource.pattern( "webhdfs/v1/**?**" );
    context.contributeChain( service, resource, chain, params );
  }

<topology>
  <gateway>
    <provider/>
    <chain name="">
      <provider role="" name="">
        <param name="" value=""/>
      </provider>
      <provider/>
    </chain>
  </gateway>
</topology>