Rename snippets directory to autoinstall_snippets

This makes directory name more consistent with
autoinstall_templates directory.
Improve names of methods which validate autoinstall templates/snippets
Improve automatic installation files validator

- Rename class and methods names to reflect better their purpose
- Improve methods documentation
- Change methods order, put dependencies first
- Allow templating validation if ksvalidator is not installed
- Eliminate test which checks if automatic installation file is local
  (all are since patch which removed remote kickstart feature)
Restore nsupdate_delete_system_pre code

This was updated by accident in a previous patch
Create automatic installation files manager

Automatic installation files management was split in several
files in Cobbler: utils, remote, api, action_validate, validate.
Concentrate this functionality in only two files (autoinstallgen
and autoinstall_manager) to make code easier to understand and
maintain.
Read autoinstall templates and snippets from settings

Remove all autoinstall templates/snippets directories
hardcoded references from code.
Merge pull request #1288 from alanoe/alanoe/refactor_autoinstall_templates_snippets

Alanoe/refactor autoinstall templates snippets
several pep8 fixes.

cobbler/api.py:30: redefinition of unused 'action_buildiso' from line 29
cobbler/autoinstall_manager.py:270: local variable 'server' is assigned to but never used
cobbler/autoinstall_manager.py:295: local variable 'failed' is assigned to but never used
cobbler/item_image.py:26: 'validate' imported but unused
cobbler/remote.py:46: 'validate' imported but unused
cobbler/validate.py:21: 'os' imported but unused
Merge pull request #1292 from jmaas/master

several pep8 fixes.
[koan] Use new dracut ip option for configuring static interfaces
Add script to create a KVM guest

Also, the script installs a Red Hat based Linux distribution
manually or in automated fashion using a kickstart.

The script will be used for creating a Cobbler test environment
in automated fashion.

Also, add a simple Cent OS 6.5 kickstart so it can be used to
easily create a test environment. url and possibly network kickstart
directives will be replaced when script which creates a KVM guest
is run.
Add script to automate installation from source code

Tasks:
 * optionally add yum repositories
 * install required dependencies
 * install proftpd FTP server
 * install ISC DHCP server
 * install fence-agents
 * install cobbler

Supported architectures: x86, ppc64
Supported Linux distributions: CentOS and RHEL Server 6/7

extra author: Andrea Vidigal Bucci <andreavb@linux.vnet.ibm.com>
Merge pull request #1294 from opoplawski/static

[koan] Use new dracut ip option for configuring static interfaces
Merge pull request #1298 from alanoe/alanoe/add_script_create_kvm_guest

Add script to create a test KVM guest
Merge pull request #1299 from alanoe/alanoe/automate_installation_from_source

Add script to automate installation from source code
Add proxy support for get-loaders, signature update and reposync.

This patch fixes #1286.
Add proxy support for get-loaders, signature update and reposync.

This patch fixes #1286.
Merge pull request #1301 from jmaas/master

Add proxy support for get-loaders, signature update and reposync.
Changed Apache configuration directory in Ubuntu 14.04 (#1208).
Merge pull request #1303 from jmaas/master

Changed Apache configuration directory in Ubuntu 14.04 (#1208).