playbook: remove invalid certificate in slapos.cfg

We need to remove invalid signature certificate for networkcache from current slapos.cfg file because it prevent slapos to download compiled SR from networkcache.

For more information about this problem, see: https://lab.nexedi.com/nexedi/slapos/commit/04e5d4aa25ea9f3a4d69d6ae240ac747fcfe5819