nova-notifs: allow to use secure keystone endpoint

When getting the nova_admin_tenant_id, the provider is now able to
connect to Keystone by using a SSL endpoint.

Closes-bug #1334250

Change-Id: Ifa3399ec6721c99e4610d75dfb46e570d015d112
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>