[JENKINS-19565] Pass ${it} to contents of dropdownDescriptorSelector

This is fix for [JENKINS-19565] (https://issues.jenkins-ci.org/browse/JENKINS-19565).

When the contents dynamically rendered in `dropdownDescriptorSelector`, `${it}`, which is usually a instance of the project, is not passed.

This often causes problems especially for plugins wrapping other plugins.
For example, when used with [Flexible Publish plugin] (https://wiki.jenkins-ci.org/display/JENKINS/Flexible+Publish+Plugin), [Email-ext plugin] (https://wiki.jenkins-ci.org/display/JENKINS/Email-ext+plugin) fails to display options for mult-configuration projects.

And this adds `capture` parameter to `dropdownDescriptorSelector` as that of `hetero-list`, which allows pass also user-defined variables.
I'd say that such change can be merged if nobody complains. 
Could you file an improvement/bug for the sake of the consistency?