Rally info: deploy engines & server providers

* Add support for searching for deploy engines & server providers in rally info
* Unify the docs for those classes so that all they contain config samples and
  have the same formatting style.

Usage sample:

    $ rally info find ExistingServers
    ExistingServers (server provider).

    Just return endpoints from its own configuration.

    Sample configuration:

        {
            "type": "ExistingServers",
            "credentials": [{"user": "root", "host": "localhost"}]
        }

    $ rally info find DevstackEngine
    DevstackEngine (deploy engine).

    Deploy Devstack cloud.

    Sample configuration:

        {
            "type": "DevstackEngine",
            "devstack_repo": "https://example.com/devstack/",
            "localrc": {
                "ADMIN_PASSWORD": "secret"
            },
            "provider": {
                "type": "ExistingServers",
                "credentials": [{'user': 'root', 'host': '10.2.0.8'}]
            }
        }

Change-Id: I9a4ae51d01bff155ba22c68f86666c8896eacc44
Merge "Rally info: deploy engines & server providers"