CentOS 7 Generic Cloud login?

Solution 1:

The default username for the CentOS 7 cloud image is centos. There is no password; instead you log in with the ssh key you provided at instance creation. You can then sudo and do whatever you like.

Solution 2:

If you downladed the GenericCloud image from cloud.centos.org, it does not have any users created. You can change the root password using virt-customize from libguestfs-tools package:

# virt-customize -a image_name.qcow2 --root-password password:your.great.password

Just keep in mid that this is not secure. After you login as root, you should create a sudo user, set up key-based authentication for ssh, disable root login, etc.