test_webui: Allow False values in configuration for no_ca, no_dns, has_trusts

The driver only checked if the corresponding value was in the config, so
    no_dns: False
had the same effect as
    no_dns: True

Change the check to take the value into consideration.

This makes false-y values like False (from YAML) and empty string
(from environment) work as if the value was not specified.

ipatests/test_webui/ui_driver.py

@@ -241,19 +241,19 @@ class UI_driver(object):
         """
         FreeIPA server was installed with CA.
         """
-        return 'no_ca' not in self.config
+        return not self.config.get('no_ca')
 
     def has_dns(self):
         """
         FreeIPA server was installed with DNS.
         """
-        return 'no_dns' not in self.config
+        return not self.config.get('no_dns')
 
     def has_trusts(self):
         """
         FreeIPA server was installed with Trusts.
         """
-        return 'has_trusts' in self.config
+        return self.config.get('has_trusts')
 
     def has_active_request(self):
         """