Minor code Improvements

[darxriggs]

These are just some minor code improvements.

[oleg-nenashev]

Actually, please revert the getActiveInstance() changes. Behavior of getInstance() changes was changed few times in incompatible way, better to use Jenkins.get() which will be available after 2.138.x

[darxriggs]

I just removed the last commit containing the getActiveInstance() changes.

[oleg-nenashev]

I should have fixed CI now, let's see. https://groups.google.com/forum/#!topic/jenkinsci-dev/v9d-XosOp2s

[darxriggs]

I think I am nearly done for now with changes.

Just two more points:

1. Please have a look at https://github.com/jenkinsci/custom-tools-plugin/branches and consider removing the stale branches.
2. The code has a lot of trailing whitespace noise. Shall I prepare a pull request cleaning this up? Or do you fear a) too many merge conflicts with open pull request and b) cumbersome navigation when it comes to git blame?