op-mode: T6480: must call pki.py helper as root to work with ACME certificates

This is an addition to commit 65fba1cd2 ("op-mode: T6377: must call pki.py
helper as root to work with ACME certificates") which missed out the basic
"show pki" command, as the <command> XML node was deep down in the view.

(cherry picked from commit 9456113a202f98a777d44c756f7f51c95b1df027)