terraform: Add parameter for plan file

Add terraform_plan parameter to be defined when using apply
command. It allows to specify a plan file to apply.

Change-Id: I2858317e4dd560030a367b2cd2ac43db5585152a

roles/terraform/README.rst

@@ -17,6 +17,10 @@ Run terraform command. Assumes the appropriate version of terraform has been ins
 
    String of extra command line options to pass to terraform.
 
+.. zuul:rolevar:: terraform_plan
+
+   Optional. Path to the plan file to use when using 'apply' command.
+
 .. zuul:rolevar:: terraform_workspace
 
    Name of the workspace to operate against.

roles/terraform/defaults/main.yaml

@@ -1,5 +1,6 @@
 terraform_executable: "{{ ansible_user_dir }}/.local/bin/terraform"
 terraform_extra_args: ""
+terraform_plan: ""
 terraform_create_workspace: false
 terraform_purge_workspace: false
 terraform_comment: true

roles/terraform/tasks/main.yaml

@@ -59,6 +59,7 @@
     -input=false
     {{ terraform_extra_args }}
     {% if terraform_command == 'apply' or terraform_command == 'destroy' %}-auto-approve{% endif %}
+    {{ terraform_plan }}
   args:
     chdir: "{{ zuul_work_dir }}"
   environment: