Add support for Cobertura and multi-module maven projects

Currently hudson.plugins.covcomplplot.analyzer.CoberturaMethodHandler.process assumes the filename of the cobertura coverage report is "coverage.xml". However, in the case of a multi-module maven project there could be any number of coverage files, i.e., coverage1.xml, coverage2.xml, etc...  

This commit generalizes the processing to account for any number of coverage files.