Deploying by default an Overcloud capable only of running the overcloud pingtest (naked overcloud)

The reason of this blueprint it's to show that TripleO
deployments are using more resources in the mean time
new services are added by default to the overcloud.
In which case, deploying a naked overcloud might be
useful for debugging and testing purposes.

A way to do it might be defining all optional services in the
overcloud resource registry as:
  OS::TripleO::Services::<ServiceName>: OS::Heat::None

With this change, the service <ServiceName> won't be
deployed by default in the overcloud.

Now, if <ServiceName> needs to be enabled, the resource
registry will be overriden by using:

-e environments/services/<service>.yaml which will contain:
  resource_registry:
    OS::TripleO::Services::<ServiceName>: ../../puppet/services/<service>.yaml

i.e. if it's needed to enable mistral services.
In:
  tripleo-heat-templates/overcloud-resource-registry-puppet.yaml
  resource_registry:
  ...
    OS::TripleO::Services::MistralEngine: OS::Heat::None
    OS::TripleO::Services::MistralApi: OS::Heat::None
    OS::TripleO::Services::MistralExecutor: OS::Heat::None

In:
  tripleo-heat-templates/environments/services/mistral.yaml
  resource_registry:
    OS::TripleO::Services::MistralEngine: ../../puppet/services/mistral-engine.yaml
    OS::TripleO::Services::MistralApi: ../../puppet/services/mistral-api.yaml
    OS::TripleO::Services::MistralExecutor: ../../puppet/services/mistral-executor.yaml

Now, if mistral is needed to be deployed in the overcloud, can be deployed as:

openstack overcloud deploy \
...
-e /home/stack/tripleo-heat-templates/environments/services/mistral.yaml

By definition, a naked Overcloud deployment will have only the required
components to run the overcloud pingtest.