Fix PKCS11 helper

Slots in HSM are not assigned statically, we have to chose proper
slot from token label.

Softhsm i2.2.0 changed this behavior and now slots can change over
time (it is allowed by pkcs11 standard).

Changelog:
* created method get_slot() that returns slot number from
  used label
* replaces usage of slot in __init__ method of P11_Helper
  with label
* slot is dynamically detected from token label before
  session is opened
* pkcs11-util --init-token now uses '--free' instead '--slot'
  which uses first free slot (we don't care about slot numbers
  anymore)

https://pagure.io/freeipa/issue/6692

Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>

ipaserver/dnssec/localhsm.py

@@ -89,10 +89,11 @@ class Key(collections.MutableMapping):
     def __repr__(self):
         return self.__str__()
 
+
 class LocalHSM(AbstractHSM):
-    def __init__(self, library, slot, pin):
+    def __init__(self, library, label, pin):
         self.cache_replica_pubkeys = None
-        self.p11 = _ipap11helper.P11_Helper(slot, pin, library)
+        self.p11 = _ipap11helper.P11_Helper(label, pin, library)
         self.log = logging.getLogger()
 
     def __del__(self):