Let koan support CentOS 7 version numbering scheme

Borrowing code of os_release from cobbler/utils.py
Ref: http://lwn.net/Articles/601914/
import re before using it
Remove ppc support from RHEL6 and RHEL7 distros signatures

RHEL 6/7 documentation mentions that Power PC 32 bits architecture
is not supported.
Add missing self.logger to util.die() calls.  Noted in issue #1326.
add $power_id to support lanplus for ipmilan

pass lanplus to power_id, that can suport lanplus mode in ipmitool
Merge pull request #1334 from qingbo/koan-centos-7

Let koan support CentOS 7 version numbering scheme
Merge pull request #1336 from alanoe/alanoe/remove_ppc_from_rhel67_signatures

Remove ppc support from RHEL6 and RHEL7 distros signatures
Merge pull request #1339 from opoplawski/die

Add missing self.logger to util.die() calls.  Noted in issue #1326.
Merge pull request #1341 from elvis-macak/master

add $power_id to support lanplus for ipmilan
Fix distro signatures autoinstall templates paths

This should have been done in 7ef12d1b.
Merge pull request #1347 from alanoe/alanoe/fix_distro_signatures_autoinstall_paths

Fix distro signatures autoinstall templates paths
Drop specific memtest support

memtest can be added as a Cobbler image if user wants, no need to
handle it specifically in code.
Merge pull request #1351 from alanoe/alanoe/drop_memtest_support

Drop specific memtest support