alternatives: handle absent link, add integration tests (#27967)

* alternatives: add integration tests

* alternatives: handle absent link (fix AttributeError)

Error occurred at least on Debian Stretch and OpenSuse 42.2:
Traceback (most recent call last):
  File "/tmp/ansible_RY6X41/ansible_module_alternatives.py", line 161, in <module>
    main()
  File "/tmp/ansible_RY6X41/ansible_module_alternatives.py", line 113, in main
    current_path = current_path_regex.search(display_output).group(1)
AttributeError: 'NoneType' object has no attribute 'group'

update-alternatives stdout sample:
dummy - manual mode
  link best version is /usr/bin/dummy1
  link currently absent
  link dummy is /usr/bin/dummy

* alternatives: PEP 8 fixes

* alternatives: fix copyright in integration tests

* alternatives: nested loops handle more than 2 items

Thanks to Michael Scherer (@mscherer) for pointing that.

* alternatives: enable integration tests

test/integration/targets/alternatives/tasks/setup_test.yml

@@ -0,0 +1,22 @@
+- template:
+    src: dummy_alternative
+    dest: '{{ alternatives_dir }}/dummy'
+    owner: root
+    group: root
+    mode: 0644
+  when: with_alternatives or ansible_os_family != 'RedHat'
+
+# update-alternatives included in Fedora 26 (1.10) & Red Hat 7.4 (1.8) segfaults
+# when <admindir>/dummy file contains only mode and link. Hence the file is
+# deleted instead of containing only mode and link. The file below works fine with
+# newer version of update-alternatives:
+# """
+# auto
+# /usr/bin/dummy
+#
+#
+# """
+- file:
+    path: '{{ alternatives_dir }}/dummy'
+    state: absent
+  when: not with_alternatives and ansible_os_family == 'RedHat'