nextcloud: use shorter paths for test

Nextcloud's redis is causing `AF_UNIX path too long` on test nodes.
4 jobs for master in 0 seconds
Status Job ID Name Coverage
  External
failed SLAPOS-SR-TEST-MASTER

04:29:56

failed SLAPOS-SR-TEST-MASTER

05:08:03

failed SLAPOS-SR-TEST-MASTER

04:27:27

failed SLAPOS-SR-TEST-MASTER

03:53:16