Jobref calls should support overriding node filter params

It is sometimes required for a coordination job to call subordinate jobs overriding them with its node filter and dispatch flags. This is needed because each job must be defined with filter expressions and dispatch flags of its own.

A coordination job must be able to override these like is possible with the run command arguments. Eg

 run ‐I tags=dev ‐X os‐family=windows ‐C 2 ‐K ‐j ’test/Job1’ -- -optionA valA

The jobref flag might support this syntax, too.

Three hypothetical strawman examples illustrate the theme:

Ex#1) Overload arg line attribute using the double dash as delimiter:

 <jobref group="test" name="Job1"/>            
     <arg line="‐I tags=dev ‐X os‐family=windows ‐C 2 ‐K -- -optionA valA"/>                                            
 </jobref>

Ex#2) Allow nodefilters and dispatch tags as nested elements:

<jobref group="test" name="Job1"/>            
    <arg line="-optionA valA"/>    
    <nodefilters excludeprecedence="true">                                                               
      <include>
        <os-family>windows</os-family>
      </include>
      <exclude>
        <tags>dev</tags>
      </exclude>
    </nodefilters>
    <dispatch>
      <threadcount>2</threadcount>
      <keepgoing>true</keepgoing>
    </dispatch>                                        
</jobref>

Ex#3) Provide a shorthand tag that implicitly overrides the nodefilters and
dispatch configuration for the subordinate job:

<jobref group="test" name="Job1"/>            
     <override nodefilters="true" dispatch="true"/>
     <arg line="-optionA valA"/>                                            
</jobref>