Explicitly setting of PKI token signing cert

Enable setting of the cert, key, and cacert for PKI token signing.

Replaces abandoned change https://review.openstack.org/#/c/93598

Change-Id: I91f1c3c6f2d1fcb477699b2b23b56cbe22006d4e
Closes-Bug: #1319470
Merge "Explicitly setting of PKI token signing cert"