[FIXED JENKINS-17858] allow <f:dropdownDescriptorSelector> to be defaulting to an instance

fixes https://issues.jenkins-ci.org/browse/JENKINS-17858 and is a prerequisite to https://issues.jenkins-ci.org/browse/JENKINS-17723, which I'll send an other request shortly

In addition to a default descriptor, this change allows to define a default instance in a <f:dropdownDescriptorSelector>