Modifying default variables and config

Renamed variables for defaults:
- flatpak_method -> default_flatpak_method: system
- defaults.paths.prefix -> default_install_prefix: /usr/local
New variables:
- default_install_method: package

tasks/helpers/flatpak.yml

@@ -0,0 +1,9 @@
+# vim: set filetype=yaml.ansible :
+---
+- name: Install flatpak
+  become: "{{ ext_become }}"
+  community.general.flatpak:
+    method: "{{ pkg_method }}"
+    remote: "{{ pkg_remote }}"
+    name: "{{ pkg_name }}"
+    state: present