centos, rhel, oracle: remove previous kernels to minimize image size

[ceetav]

yum update keeps several kernel versions installed at the same time by default (installonly_limit).
So, bento/centos-6.9 image v201803.24.0 has two installed kernels: kernel-2.6.32-696.el6.x86_64 and kernel-2.6.32-696.23.1.el6.x86_64 .
Removal of the unused one would reduce the image size by about 50 megs.

1. The proposal is to delete via package-cleanup from yum-utils, which probably should take care for kernel-uek in the Oracle Linux.

2. Alternative approach could be direct manipulation:

   unused_kernels=("`rpm -q kernel kernel-uek | grep -v "\`uname -r\`"`")
   if [ ${#unused_kernels[@]} -gt 0 ]; then yum remove -y ${unused_kernels[@]}; fi

bento/centos-6.9 v201803.24.0 kernels

[vagrant@localhost ~]$ rpm -qi kernel
Name        : kernel                       Relocations: (not relocatable)
Version     : 2.6.32                            Vendor: CentOS
Release     : 696.el6                       Build Date: Tue 21 Mar 2017 07:56:16 PM UTC
Install Date: Sat 24 Mar 2018 02:35:41 PM UTC      Build Host: c1bm.rdu2.centos.org
Group       : System Environment/Kernel     Source RPM: kernel-2.6.32-696.el6.src.rpm
Size        : 137522245                        License: GPLv2
Signature   : RSA/SHA1, Thu 23 Mar 2017 03:02:10 PM UTC, Key ID 0946fca2c105b9de
Packager    : CentOS BuildSystem <http://bugs.centos.org>
URL         : http://www.kernel.org/
Summary     : The Linux kernel
Description :
The kernel package contains the Linux kernel (vmlinuz), the core of any
Linux operating system.  The kernel handles the basic functions
of the operating system: memory allocation, process allocation, device
input and output, etc.

Name        : kernel                       Relocations: (not relocatable)
Version     : 2.6.32                            Vendor: CentOS
Release     : 696.23.1.el6                  Build Date: Tue 13 Mar 2018 11:01:52 PM UTC
Install Date: Sat 24 Mar 2018 02:36:48 PM UTC      Build Host: x86-01.bsys.centos.org
Group       : System Environment/Kernel     Source RPM: kernel-2.6.32-696.23.1.el6.src.rpm
Size        : 139542045                        License: GPLv2
Signature   : RSA/SHA1, Wed 14 Mar 2018 02:39:08 PM UTC, Key ID 0946fca2c105b9de
Packager    : CentOS BuildSystem <http://bugs.centos.org>
URL         : http://www.kernel.org/
Summary     : The Linux kernel

[cheeseplus]

This looks slick and we'll just need to run it through some light tests to make sure nothing breaks but I think this'll get merged post ChefConf :D