partial fix for JENKINS-17913 expand variables for branch
partial fix for JENKINS-17913 expand variables for branch

JENKINS-17913 asked for being able to expand variables for certain
configuration properties I have added expansion for the specified branch
which allows users to choose a branch to checkout and pass it in as a
variable

Typz/repo-plugin,Typz/repo-plugin,jenkinsci/repo-plugin,maksonlee/repo-plugin,mikaukora/repo-plugin,solganik/repo-plugin,mikaukora/repo-plugin,jenkinsci/repo-plugin,solganik/repo-plugin,maksonlee/repo-plugin