A standalone mistral server configured for an external OpenStack

It should be possible to install a mistral server which is configured to manage on an external OpenStack compatible cloud.

We can execute a workflow within the cloud that the client requests. It will require
the following changes.

* Support password authentication:
     Add alternative authentication middleware to authenticate against Keystone using
     username and password instead of validating existing auth token. The benefit
     being that you no longer require admin/service token to authenticate users.

     To use, update mistral.conf
         flavor = standalone
    We can refer heat's KeystonePasswordAuthProtocol[1] to implement it.

* python-mistralclient on what credentials to send in standalone mode

[1] https://github.com/openstack/heat/blob/master/heat/common/auth_password.py