Don't use hostvars to set the metadata_agent.ini variables

Currently variables in group_vars in ansible won't be expanded when
specified via "hostvars". To get around this we can template using the
actual varaibles themselves.

* Change hostvars "nova_metadata_proxy_shared_secret" to use
  "nova_metadata_proxy_secret"
* Change hostvars "nova_metadata_ip" to use "internal_vip_address"

Fixes #16