* 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.