Allow to use trusts and application credentials with Openstack driver

[djuarezg]

The library you are using, gophercloud, already allows this:

• https://github.com/gophercloud/gophercloud/tree/master/openstack/identity/v3/extensions/trusts
• https://github.com/gophercloud/gophercloud/tree/master/openstack/identity/v3/applicationcredentials

It would be great to have these two auth methods for the Openstack driver for docker-machine.