Shrink lizard's jenkins-data LV

Originally created by @intrigeri on #12119 (Redmine)

As noted on #10396 (closed) already, and rediscovered today while doing #11806 (closed), we use about half of the space allocated to that LV. My estimates say that we could shrink it down to 300GB and still be fine until the end of 2018.

Related issues

Edited by intrigeri