Fail on unknown Command options

When unknown keyword arguments are passed to a Command, raise an
error instead of ignoring them.

Options used when IPA calls its commands internally are listed
in a new Command attribute called internal_options, and allowed.

Previous patches (0b01751c, c45174d6, c5689e7f) made IPA not use
unknown keyword arguments in its own commands and tests, but since
that some violations were reintroduced in permission_find and tests.
Fix those.

Tests included; both a frontend unittest and a XML-RPC test via the
ping plugin (which was untested previously).

https://fedorahosted.org/freeipa/ticket/2509

tests/test_xmlrpc/test_host_plugin.py

@@ -126,7 +126,7 @@ class test_host(Declarative):
             command=('host_add', [fqdn1],
                 dict(
                     description=u'Test host 1',
-                    locality=u'Undisclosed location 1',
+                    l=u'Undisclosed location 1',
                     force=True,
                 ),
             ),