Allow disabling process killing on interruption (#3293)

* Allow disabling process killing on interuption

Plugins may want to do custom action when the execution of a remote process is interrupted. Currently it is not possible, because
the process started on a slave is killed on interruption.
My actual use case: https://groups.google.com/forum/#!msg/jenkinsci-dev/s6IkynOzdqE/2eFNKN9lAAAJ

My solution is to have a flag called killWhenInterrupted, set by default to true, which prevents process killing on the slave when unset.

* Invert killWhenInterrupted flag

* Add javadoc