Change tox-siblings disablement to allow opt-in

The relevant section is already protected by a variable. Instead of
commenting out the call, set the default value of the flag to false.
This allows people who need the functionality and are not broken by the
existing issue to use it.

Change-Id: Ifa5cc3e335b9e9ad52c4c5c9ab71aaea70d4c9aa

roles/tox-siblings/defaults/main.yaml

@@ -1,6 +1,7 @@
 ---
 tox_envlist: venv
 tox_executable: tox
-tox_install_siblings: true
+# NOTE(jeblair) disabled because of lack of constraints support
+tox_install_siblings: false
 
 zuul_work_dir: "{{ zuul.project.src_dir }}"

roles/tox-siblings/tasks/main.yaml

@@ -10,10 +10,9 @@
   when: tox_install_siblings
 
 # TODO(mordred) handle tox_envlist being a list
-# NOTE(jeblair) disabled because of lack of constraints support
-#- name: Install any sibling python packages
-#  tox_install_sibling_packages:
-#    tox_envlist: "{{ tox_envlist }}"
-#    project_dir: "{{ zuul_work_dir }}"
-#    projects: "{{ zuul.projects | selectattr('required') | list }}"
-#  when: tox_install_siblings
+- name: Install any sibling python packages
+  tox_install_sibling_packages:
+    tox_envlist: "{{ tox_envlist }}"
+    project_dir: "{{ zuul_work_dir }}"
+    projects: "{{ zuul.projects | selectattr('required') | list }}"
+  when: tox_install_siblings