Honor constraints files in tox-siblings

We use the constraints file when we do the actual sibling installation,
but not when we do the initial no-test install. That could lead to
something being installed at the wrong version and not getting updated
subsequently.

Shifted the siblings logic into a file that's conditionally included so
we can avoid a bunch of if defined additions.

Change-Id: I4e71ab2ea8e167f22c3a66b95dc6f898a35f0284

roles/tox-siblings/tasks/main.yaml

@@ -1,19 +1,3 @@
-- name: Require tox_envlist variable
-  fail:
-    msg: tox_envlist is required for this role
-  when: tox_envlist is not defined
-
-- name: Run tox without tests
-  command: "{{ tox_executable }} --notest -e{{ tox_envlist }}"
-  args:
-    chdir: "{{ zuul_work_dir }}"
-  when: tox_install_siblings
-
-# TODO(mordred) handle tox_envlist being a list
-- name: Install any sibling python packages
-  tox_install_sibling_packages:
-    tox_envlist: "{{ tox_envlist }}"
-    tox_constraints_file: "{{ tox_constraints_file | default(omit) }}"
-    project_dir: "{{ zuul_work_dir }}"
-    projects: "{{ zuul.projects | selectattr('required') | list }}"
+- name: Install tox siblings
+  include: siblings.yaml
   when: tox_install_siblings

roles/tox-siblings/tasks/siblings.yaml

@@ -0,0 +1,38 @@
+- name: Require tox_envlist variable
+  fail:
+    msg: tox_envlist is required for this role
+  when: tox_envlist is not defined
+
+- name: Check to see if the constraints file exists
+  stat:
+    path: "{{ tox_constraints_file }}"
+    get_checksum: false
+    get_mime: false
+    get_md5: false
+  register: stat_results
+  when: tox_constraints_file is defined
+
+- name: Fail if constraints file is missing
+  when: tox_constraints_file is defined and not stat_results.stat.exists
+  fail:
+    msg: tox_constraints_file is defined but was not found
+
+- name: Record file location
+  set_fact:
+    tox_constraints_env:
+      UPPER_CONSTRAINTS_FILE: "{{ tox_constraints_file }}"
+  when: tox_constraints_file is defined
+
+- name: Run tox without tests
+  command: "{{ tox_executable }} --notest -e{{ tox_envlist }}"
+  args:
+    chdir: "{{ zuul_work_dir }}"
+  environment: "{{ tox_environment|combine(tox_constraints_env|default({})) }}"
+
+# TODO(mordred) handle tox_envlist being a list
+- name: Install any sibling python packages
+  tox_install_sibling_packages:
+    tox_envlist: "{{ tox_envlist }}"
+    tox_constraints_file: "{{ tox_constraints_file | default(omit) }}"
+    project_dir: "{{ zuul_work_dir }}"
+    projects: "{{ zuul.projects | selectattr('required') | list }}"