Remove keystone version in tacker.conf

Tacker is failing registering VIM monitoring
into mistral because the v3 in the auth url.

The auth url is being used auth_url + '/v3' in the code,
ending in a http://url:35357/v3/v3

https://github.com/openstack/tacker/blob/210d8f93bb5e7e179bff176d98d3079ab7f33ca0/tacker/nfvo/nfvo_plugin.py#L93


Change-Id: If5abbee1377ecdb2913a42bdfa77e7fbbfbe9367