Web UI integration tests: Compute range sizes to avoid overlaps

Heavily inspired by code from xmlrpc tests.

To obtain ranges, this patch also adds method to execute FreeIPA command through Web UI.
It uses Web UI instead of ipalib so it doesn't need to care about authentication on a test-runner machine.

https://fedorahosted.org/freeipa/ticket/3744

ipatests/test_webui/ui_driver.py

@@ -489,6 +489,31 @@ class UI_driver(object):
             }
         return info
 
+    def execute_api_from_ui(self, method, args, options, timeout=30):
+        """
+        Executes FreeIPA API command/method from Web UI
+        """
+        script = """
+        var method = arguments[0];
+        var args = arguments[1];
+        var options = arguments[2];
+        var callback = arguments[arguments.length - 1];
+        var IPA = require('freeipa/ipa');
+
+        var cmd = IPA.command({
+            method: method,
+            args: args,
+            options: options,
+            on_success: callback,
+            on_error: callback
+        });
+
+        cmd.execute();
+        """
+        self.driver.set_script_timeout(timeout)
+        result = self.driver.execute_async_script(script, *[method, args, options])
+        return result
+
     def click_on_link(self, text, parent=None):
         """
         Click on link with given text and parent.