Remove support for CentOS 7 and Python 2

* Always use Python 3
* Drop code paths for CentOS 7
* Drop support for Yum
* Remove support for host NTP daemon, always use chrony
* Switch references from 'yum_install_epel' to 'dnf_install_epel'
* Remove overcloud host image workaround for tagged VLAN admin network
* Remove the kayobe.utils.yum_install function, which is unused

Change-Id: I368f6edafed9779658798fc342116b4c1b3ffd48
Story: 2006574
Task: 39481