CI: stop using zuul as kayobe_ansible_user in TLS jobs

Previously we were using the zuul user in the TLS jobs. This was due to
a permissions issue when accessing the CA certificate in kayobe-config
in the zuul user's home directory.

This change reverts to the default of using the stack user for the TLS
jobs. In order to make this work, the generated CA cert chain is added
to the trust store.

Change-Id: I875f8976df75dee68ba00842fe624c29cc1b123c