Create tox_environment to allow users access to shell variables

Because we run tox using the shell task, it is possible users will
need to pass custom shell variables to their tox.ini files. Expose a
way for users to do just that.

Change-Id: I0a0c15d85d91fe0a23cf688f0b0f228133922264
Signed-off-by: Paul Belanger <pabelanger@redhat.com>

roles/tox/tasks/main.yaml

@@ -2,6 +2,7 @@
   args:
     chdir: "{{ zuul_work_dir }}"
     executable: /bin/bash
+  environment: "{{ tox_environment | default(omit) }}"
   shell: |
     # If a bundle file is present, call tox with the jenkins version of
     # the test environment so it is used.  Otherwise, use the normal

zuul.yaml

@@ -20,11 +20,14 @@
       This performs basic host and general project setup tasks common
       to all tox unit test jobs.
 
-      Responds to two variables:
+      Responds to three variables:
 
       tox_envlist
         work against specified environments (ALL selects all)
 
+      tox_environment
+        list of environmental variables to pass to bash shell
+
       tox_command_line
         an optional command line
     run: playbooks/tox/run