SM: rectify private_key check

Closes-bug: #1641014

if user re-adds json with ssh_public_key, it causes ssh_private_key
to reset to None

Change-Id: Ic52ee5880873e4a04c93ec272331fa1fd3dbfb49

src/server_mgr_db.py

@@ -809,7 +809,7 @@ def modify_server(self, server_data):
                 for k,v in server_tags.iteritems():
                     server_data[rev_tags_dict[k]] = v
 
-            if "ssh_private_key" and "ssh_public_key" in server_data:
+            if "ssh_private_key" in server_data and "ssh_public_key" in server_data:
                 private_key = str(server_data.pop("ssh_private_key", None))
                 public_key = str(server_data.pop("ssh_public_key", None))
                 server_data["ssh_private_key"] = private_key