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.