Multi region support

mistral should be able to work as part of a multi region setup.
missing capabilities:
1. Keystone support : The ability to ask from keystone an end point by its region
2.region as a task property:

from Vijayaguru Guruchave (https://blueprints.launchpad.net/mistral/+spec/mistral-multicloud):

It allows mistral to run without any configured keystone endpoint. All actions will be performed using whatever keystone endpoint and credentials that are passed to the request.

It will be possible to execute a workflow within the clouds that the client requests. We can refer heat blueprint[1] to implement the following.

    * An [authpassword] configuration section in mistral.conf with the following options:
        multi_cloud - boolean defaults to False. When True the configured auth_uri will be ignored and the X-Auth-Url header included in the client request will be used instead
        allowed_auth_uris - list defaults to []. Ignored if multi_cloud is False. If empty, authenticating against any supplied X-Auth-Url will be attempted. Otherwise the X-Auth-Url will be validated against the configured list and authentication will fail if no match is found.

It should then be possible to launch workflows on arbitrary clouds.

[1] https://wiki.openstack.org/wiki/Heat/Blueprints/heat-multicloud