don't override plotlyServerURL if one is set in iplot/plot (#1615)

* only set plotlyServerURL using the plotly_domain in the .config file
if one isn't specified in the config dict passed to iplot/plot
* make it clear that the default in clean_config.get is None

plotly/offline/offline.py

@@ -161,7 +161,8 @@ def _get_jconfig(config):
 
     plotly_platform_url = plotly.tools.get_config_plotly_server_url()
 
-    clean_config['plotlyServerURL'] = plotly_platform_url
+    if not clean_config.get('plotlyServerURL', None):
+        clean_config['plotlyServerURL'] = plotly_platform_url
 
     if (plotly_platform_url != 'https://plot.ly' and
         clean_config.get('linkText', None) == 'Export to plot.ly'):