Auth fails with SSL exception connecting to keystone over https

Having set up keystone to operate locally over https I cannot get python-mistralclient to authenticate.

mistral --os-cacert ca.pem --os-password secrete --os-username admin --os-tenant-name demo --os-auth-url "https://localhost:5000/v2.0"
Authorization Failed: SSL exception connecting to https://localhost:5000/v2.0/tokens

I've also tried with the environment variables that I would set for the keystone client if I was to use it directly. Looking into it I may well be wrong but I cannot see how the ca cert is passed into the python-keystoneclient that underpins keystone interactions.