Add Ceph dashboard as composable role

As ceph-dashboard is available on Ceph, the idea is to consider a user scenario in which the dashboard is deployed along with the other ceph components using TripleO.
For this reason it's useful to create a new composable service as it's been done with the others components to get the ceph dashboard deployment automated.

For example, given a deployment with the following components:

* mons
* osds

* ( ... )

* dashboard (new component to be enabled)

this use case allows operators to include a ceph-dashboard template containing all the parameters (and resources) needed to enable the dashboard when the ceph-ansible playbook execution occur (having OS::TripleO::Services::CephDashboard in the same way as we have the OS::TripleO::Services::CephOSD).

The described approach allows this new component to be treated as a composable service just like the OSD, Mon and the other ceph components.

We will also impose two other constraints:

* It's a role restricted to the control plane network;

* It's disabled by default and can be enabled adding the environments/ceph-ansible/ceph-dashboard.yaml file;

To enable this blueprint there is a dependency on [1], so in the very first phase is really important to ensure that the dashboard could be enabled running the ceph-ansible playbook and passing the appropriate extra variables.

[1] https://github.com/ceph/ceph-ansible/pull/3426