Do a yum update on the system.

        :param allow_reboot: If True and if a new kernel has been installed,
        the system will be rebooted

def yum_update(self, allow_reboot=False):
        """Do a yum update on the system.

        :param allow_reboot: If True and if a new kernel has been installed,
        the system will be rebooted
        """
        self.run('yum clean all')
        self.run('test -f /usr/bin/subscription-manager && subscription-manager repos --list-enabled',
                 ignore_error=True)
        self.run('yum repolist')
        self.run('yum update -y --quiet', retry=3)
        # reboot if a new initrd has been generated since the boot
        if allow_reboot:
            self.run('grubby --set-default $(ls /boot/vmlinuz-*.x86_64|tail -1)')
            default_kernel = self.run('grubby --default-kernel')[0].rstrip()
            cur_kernel = self.run('uname -r')[0].rstrip()
            if cur_kernel not in default_kernel:
                self.run('reboot', ignore_error=True)
            self.ssh_pool.stop_all()