shared: client certificates are secret properties

Mark client certificates as secret properties. Keep CA certificates as
non-secret because they are public by definition.

https://bugzilla.redhat.com/show_bug.cgi?id=1702199
12 jobs for bg/rh1702199 in 33 minutes and 6 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #256016
allowed to fail
checkpatch

00:01:46

passed #256023
t_debian:9

00:17:40

passed #256025
t_debian:sid

00:17:36

manual #256024
allowed to fail manual
t_debian:testing
passed #256017
t_fedora:28

00:23:49

passed #256018
t_fedora:29

00:32:41

failed #256019
t_fedora:30

00:00:51

manual #256020
allowed to fail manual
t_fedora:rawhide
passed #256021
t_ubuntu:16.04

00:16:47

passed #256022
t_ubuntu:18.04

00:17:08

 
  External
failed NetworkManager test verification

failed NetworkManager test verification

06:11:51

 
Name Stage Failure
failed
t_fedora:30 Test
$ date '+%Y%m%d-%H%M%S'; NM_INSTALL="dnf install -y" ./contrib/fedora/REQUIRED_PACKAGES
20190423-092705
Fedora Modular 30 - x86_64 2.6 MB/s | 2.1 MB 00:00
Fedora Modular 30 - x86_64 - Updates 878 B/s | 257 B 00:00
Fedora Modular 30 - x86_64 - Test Updates 4.2 MB/s | 2.9 MB 00:00
Fedora 30 - x86_64 - Test Updates 3.6 MB/s | 16 MB 00:04
Fedora 30 - x86_64 - Updates 674 B/s | 257 B 00:00
./contrib/fedora/REQUIRED_PACKAGES: line 13: 11 Segmentation fault (core dumped) $NM_INSTALL "$@"
ERROR: Job failed: exit code 1