sysdb: sanitize certmap rule name before using it in DN
The name of a certificate mapping and matching rule might contain
characters which are not allowed in RDNs an must be escaped before if
can be used in the DN of the cached certmap object.
Resolves: https://pagure.io/SSSD/sssd/issue/3721
Reviewed-by: Pavel Březina <pbrezina@redhat.com>