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
fixed issue where selecting a branch for manual build no longer worked
fixed check style issues
improved parameter merging

- now build parameters for the current build will override
  defaults from the project configuration
Merge pull request #19 from rburgst/branch-expansion

Branch expansion JENKINS-17913