Switch to devstack external plugin

Devstack project has two ways of extension with some additional projects support:

1) using "extras-d-hooks":
http://docs.openstack.org/developer/devstack/plugins.html#extras-d-hooks

2) using "Externally Hosted Plugins":
http://docs.openstack.org/developer/devstack/plugins.html#externally-hosted-plugins

In former it is required to copy-paste two files into the devstack dir before installation. In latter we just need to properly set up devstack local[rc] file to be able to setup some additional projects.

Details:
- https://github.com/openstack-dev/devstack/blob/master/FUTURE.rst
- http://lists.openstack.org/pipermail/openstack-dev/2015-February/056257.html

Currently, Mistral uses first approach. We need to switch to newer one that removes redundant operations to be done before installation by devstack users.