Revision 545283

OPEN - issue MNG-2577: Allow interpolation of Properties in settings.xml 
http://jira.codehaus.org/browse/MNG-2577

Modifying to use request properties (embeddable system properties), and organizing this with the existing use of envars. NOT allowing profile-property interpolation, as these could influence profiles in a recursive manner and make their values hard to determine...besides, model-interpolation will pickup profile properties before POMs are interpolated, so the only thing this will miss is profile/properties -> servers, etc. inside the settings itself.