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
2 files changed