Remove overzealous argument checking

There are situations where public_key and signing_key can be used
together, e.g. when signing a cert with another cert on the same machine:

cert:
  x509.certificate_managed:
    - name: /etc/pki/www.crt
    - public_key: /etc/pki/www.key
    - signing_cert: /etc/pki/root.crt
    - signing_private_key: /etc/pki/root.key
    - CN: 'www.local'
    - subjectAltName: 'IP:127.0.0.1'

salt/states/x509.py

@@ -557,11 +557,7 @@ def certificate_managed(name,
 
     if 'public_key' not in kwargs and 'signing_private_key' not in kwargs:
         raise salt.exceptions.SaltInvocationError(
-            'Either public_key or signing_private_key must be specified.')
-
-    if 'public_key' in kwargs and 'signing_private_key' in kwargs:
-        raise salt.exceptions.SaltInvocationError(
-            'Either public_key or signing_private_key must be specified, not both.')
+            'public_key or signing_private_key must be specified.')
 
     ret = {'name': name,
        'result': False,