Fixed error where the configuration of project/global rules didn't stick...

.... The problem was caused due to looking for a JSON key on the JSON top level instead of in the sub object.

For example configuring to use a global rule for a job did not work. Using the JSON:

{"unstableOnWarning":false,"failBuildOnError":false,"showGraphs":true,"useProjectRule":{"value":"false","parsingRulesPath":"/usr/share/jenkins/log_parser_rules/erlang_dialyzer.rules"},"stapler-class":"hudson.plugins.logparser.LogParserPublisher","kind":"hudson.plugins.logparser.LogParserPublisher"}

The old code looked at the top-level JSON for a "parsingRulesPath" and did not find any. This change makes sure 
that to look for the key in 

{"value":"false","parsingRulesPath":"/usr/share/jenkins/log_parser_rules/erlang_dialyzer.rules"}