find_entry_by_attr() should fail if multiple entries are found

It will only ever return one entry so if more than one are found
then we raise an exception. This is most easily seen in the host
plugin where we search on the server shortname which can be the
same across sub-domains (e.g. foo.example.com &
foo.lab.example.com).

https://fedorahosted.org/freeipa/ticket/1388

ipalib/errors.py

@@ -1210,6 +1210,23 @@ class AttrValueNotFound(ExecutionError):
     format = _('%(attr)s does not contain \'%(value)s\'')
 
 
+class SingleMatchExpected(ExecutionError):
+    """
+    **4027** Raised when a search should return a single match
+
+    For example:
+
+    >>> raise SingleMatchExpected(found=9)
+    Traceback (most recent call last):
+      ...
+    SingleMatchExpected: The search criteria was not specific enough. Expected 1 and found 9.
+    """
+
+    errno = 4027
+    rval = 1
+    format = _('The search criteria was not specific enough. Expected 1 and found %(found)d.')
+
+
 class BuiltinError(ExecutionError):
     """
     **4100** Base class for builtin execution errors (*4100 - 4199*).