Replace exit() by sys.exit()

exit() is used for interactive shell, it relies on the site module.
sys.exit() is considered good to use in programs.

Trivial-fix

Change-Id: I9e6e7a53581abefbb6cc3ba6ee6b853cd430d456
diff --git a/tempest/cmd/verify_tempest_config.py b/tempest/cmd/verify_tempest_config.py
index 5d867df..bc06630 100644
--- a/tempest/cmd/verify_tempest_config.py
+++ b/tempest/cmd/verify_tempest_config.py
@@ -157,7 +157,7 @@
 
     if service not in extensions_client:
         print('No tempest extensions client for %s' % service)
-        exit(1)
+        sys.exit(1)
     return extensions_client[service]
 
 
@@ -170,7 +170,7 @@
     }
     if service not in extensions_options:
         print('No supported extensions list option for %s' % service)
-        exit(1)
+        sys.exit(1)
     return extensions_options[service]