dcache-webadmin: change output field of cell admin page to monospace font

Many of the dCache services format the output into columns and that only works with a mono space font.

Target: master
Request: 2.11
Request: 2.10
Request: 2.9
Request: 2.8
Request: 2.7
Request: 2.6
Acked-by: Gerd
Acked-by: Paul
Require-notes: no
Require-book: no