p11_child: allow verification with no_verification option
Currently p11_child rejects the verification of a certificate if the
'no_verification' option is set. As a result the ssh responder does not
work as expected if 'no_verification' is set.
Related to https://pagure.io/SSSD/sssd/issue/4121
Reviewed-by: Tomáš Halman <thalman@redhat.com>