Openstack cwd (#7643)

* terraform/openstack: Use path.root for ansible_bastion_template.txt

The path.root variable points to the root module path. Using this
instead of a relative path makes less assumptions about the current
working directory.

* terraform/openstack: Add group_vars_path variable

Previously, the group_vars path was assumed to be in CWD. The
default value for the group_vars_path variable is still relative
to CWD and thus should be backwards compatible if unset.

contrib/terraform/openstack/modules/compute/variables.tf

@@ -151,3 +151,7 @@ variable "image_master" {
 variable "image_master_uuid" {
   type = string
 }
+
+variable "group_vars_path" {
+  type = string
+}