Create pkey-only option for find commands

New option --pkey-only is available for all LDAPSearch based classes
with primary key visible in the output. This option makes LDAPSearch
commands search for primary attribute only.

This may be useful when manipulating large data sets. User can at
first retrieve all primary keys in a relatively small data package
and then run further commands with retrieved primary keys.

https://fedorahosted.org/freeipa/ticket/1262

tests/test_xmlrpc/test_user_plugin.py

@@ -195,6 +195,26 @@ class test_user(Declarative):
         ),
 
 
+        dict(
+            desc='Search for %r with pkey-only=True' % user1,
+            command=(
+                'user_find', [user1], {'pkey_only': True}
+            ),
+            expected=dict(
+                result=[
+                    {
+                        'dn':lambda x: DN(x) == \
+                                DN(('uid',user1),('cn','users'),
+                                   ('cn','accounts'),api.env.basedn),
+                        'uid': [user1],
+                    },
+                ],
+                summary=u'1 user matched',
+                count=1, truncated=False,
+            ),
+        ),
+
+
         dict(
             desc='Search for %r with minimal attributes' % user1,
             command=(