[FIXED JENKINS-23498] - A new option, which disables the changelog calculation

Sometimes the SVN checkout and changelog generation procedures take much time due to the big number of <code>svn:external</code> dependencies.

Our use-case:
* checkout = 10min
* changelog calculation = 15min


The proposed option can be used to temporarily disable the calculation of changelogs in such case (e.g. for the job development purposes).