Stop reading local config for domain-specific SQL config driver

When an SQL-config-based domain-specific backend was loaded the
local config file (/etc/keystone/keystone.conf) was also read. The
local config file should not be used in this case.

This caused running the tests locally to fail if there were changes
in /etc/keystone/keystone.conf.

Change-Id: Idd095b2df375329f579c164d00dfd50b41b0e96d
Closes-Bug: 1489118

keystone/identity/core.py

@@ -194,7 +194,8 @@ class DomainConfigs(dict):
         domain_config = {}
         domain_config['cfg'] = cfg.ConfigOpts()
         config.configure(conf=domain_config['cfg'])
-        domain_config['cfg'](args=[], project='keystone')
+        domain_config['cfg'](args=[], project='keystone',
+                             default_config_files=[])
 
         # Override any options that have been passed in as specified in the
         # database.