[JENKINS-22856] Uses AbstractProject instead of Item in Descriptor.doCheckProjects

[JENKINS-22856] (https://issues.jenkins-ci.org/browse/JENKINS-22856)

It can be get confused with JobPropertyImpl.getJobOverrides or /lib/form/withCustomDescriptorByName, that is, [Cloudbees Template plugin] (http://www.cloudbees.com/jenkins-enterprise-by-cloudbees-features-templates-plugin.cb).

See following for more details:
https://groups.google.com/forum/#!topic/jenkinsci-dev/wOUBUlnE24Y