what I have done, was

yum clean all

rpm --import http://obs.kolabsys.com/repositories/Kolab:/16/CentOS_7/repodata/repomd.xml.key

and vim /etc/yum.repos.d/Kolab:16.repo

[Kolab_16]
name=Kolab 16: Stable Release (CentOS_7)
type=rpm-md
baseurl=http://obs.kolabsys.com/repositories/Kolab:/16/CentOS_7/
gpg=http://obs.kolabsys.com/repositories/Kolab:/16/CentOS_7/repodata/repomd.xml.key
gpgcheck=1
enabled=1
priority = 60

then do "yum update"

after that, I could update without asking key again