Correct bug in which Piwik site ID cannot be recorded after node is saved due to duplicate key.  Avoid registering new aliases if site has a non-positive Piwik site id.  Use more consistent key for piwik site id input on site edit form.  Use more robust method for determining piwik site id in order to register new alises and create sites.

Signed-off-by: Shai Sachs <ssachs@ngpvan.com>