Signature could not be verified for kubernetes repo
You can see very similar issue here.
As a quick workaround, you can simply disable GPG checking by setting repo_gpgcheck=0 in /etc/yum.repos.d/kubernetes.repo but this is obviously not recommended from security perspective.
Additionally you may try the following:
re-import the keys as suggested here
rpm --import https://packages.cloud.google.com/yum/doc/yum-key.gpg https://packages.cloud.google.com/yum/doc/rpm-package-key.gpginstall the following version of GNUPG as suggested here
sudo yum install -y http://mirror.centos.org/centos/7/os/x86_64/Packages/gnupg2-2.0.22-5.el7_5.x86_64.rpm