certdb: validate server cert signature
    
    PR https://github.com/freeipa/freeipa/pull/2554 added the '-e' option for CA
    cert validation. Let's also verify signature, key size, and signing algorithm
    of server certs. With the '-e' option, the installer and other
    tools will catch weak certs early.
    
    Fixes: pagure.io/freeipa/issue/7761
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Fraser Tweedale <ftweedal@redhat.com>