Change default cirros ssh user

Cirros images allow connection with root and cirros by default.
Interactive sessions for root are not allowed. This will change the
default user to cirros since it allows interactive ssh sessions.

See also https://bugs.launchpad.net/cirros/+bug/1437350

Change-Id: If929ce7a3df944bd17c7f4e3042a6aea0c318b2c
diff --git a/tempest/config.py b/tempest/config.py
index 4ee4669..19c46f6 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -996,7 +996,7 @@
                help="SSH verification in tests is skipped"
                     "for matching images"),
     cfg.StrOpt('ssh_user_regex',
-               default="[[\"^.*[Cc]irros.*$\", \"root\"]]",
+               default="[[\"^.*[Cc]irros.*$\", \"cirros\"]]",
                help="List of user mapped to regex "
                     "to matching image names."),
 ]