Do not produce unindexed search on every DEL command

Every <plugin>-del command executes an "(objectclass=*)" search
to find out if a deleted node has any child nodes which would need
to be deleted first. This produces an unindexed search for every del
command which biases access log audits and may affect performance too.

Since most of the *-del commands delete just a single object (user,
group, RBAC objects, SUDO or HBAC objects, ...) and not a tree
(automount location, dns zone, ...) run a single entry delete first
and only revert to subtree search&delete when that fails.

ipalib/errors.py

@@ -1501,6 +1501,22 @@ class BadSearchFilter(ExecutionError):
     format = _('Bad search filter %(info)s')
 
 
+class NotAllowedOnNonLeaf(ExecutionError):
+    """
+    **4210** Raised when operation is not allowed on a non-leaf entry
+
+    For example:
+
+    >>> raise NotAllowedOnNonLeaf()
+    Traceback (most recent call last):
+      ...
+    NotAllowedOnNonLeaf: Not allowed on non-leaf entry
+    """
+
+    errno = 4210
+    format = _('Not allowed on non-leaf entry')
+
+
 class CertificateError(ExecutionError):
     """
     **4300** Base class for Certificate execution errors (*4300 - 4399*).