Fix an invalid variable name

According to [1], Ansible variable names should not include hyphens.
Kolla-Ansible fails with a wrong variable name 'barbican-api' when
deploying Barbican.

This patch fixes the issue that was recently introduced in [2].

[1] http://docs.ansible.com/ansible/playbooks_variables.html#id15
[2] 08ab3d8e

Change-Id: Ib962e31ad93316e56130c9fc38dabfc918de17ce
Closes-Bug: #1703287