Restructuring/renaming DNS, DHCP, TFTP and netboot installer.

Idea: The role 'dnsmasq' and a new role 'dns-dhcp-tftp' are
drop-in replacements and interchangeable. 'netboot-installer'
works indepently of the role chosen for DNS, DHCP and TFTP.

roles/tftp-netboot-installer/handlers/main.yml

@@ -1,15 +0,0 @@
-- name: bind mount images
-  mount:
-    path: "{{ tftp_root }}/d-i/n-pkg/"
-    src: /usr/lib/debian-installer/
-    fstype: none
-    state: mounted
-    opts: bind
-  listen: bind mount images
-
-- name: rebuild di-netboot-assistant menu
-  command: "{{ item }}"
-  with_items:
-    - di-netboot-assistant rebuild-menu
-    - di-netboot-assistant rebuild-grub
-  listen: rebuild di-netboot-assistant menu