The Castellan (Barbican client) has different parameters to control
the used CA file.
This patch uses them.
Moreover, this aligns Barbican with other services by defaulting
its client config to the internal endpoint.

See also [1].

[1] https://bugs.launchpad.net/castellan/+bug/1876102

Closes-Bug: #1886615

Change-Id: I6a174468bd91d214c08477b93c88032a45c137be