Introduce role for service specs

[tomclark0]

This role provides a mechanism to write arbitrary service definitions that can be freely applied.

Currently introducing arbitrary services can be performed by using the cephadm_osd_spec and defining an alternative service_type.
Some options are not exposed with cephadm commands.

Providing a seperate role for service definitions avoids defining services with cephadm_osd_spec.

Wanted feedback if this approach is valid.

Note - There can be issues when defining both an arbitrary service and accompanying ingress service. The ingress service is defined before the backend.

If this approach is valid, then the cephadm_ingress_services role could be refactored to support the re-ordering of tasks.

[mnasiadka]

I think we'd need some discussion to lay out some plan - either we're having a role per each ,,spec group'' (e.g. osd specs, ingress specs, etc) - which doesn't sound like a good idea.
I think I'd be more inclined to go with a universal role - that would cover all sorts of specs - OSD, RGW, Ingress, NFS, whatnot - and rather have them defined in a way like we define RGW spec in cephadm role - e.g. cephadm_radosgw_services
@jackhodgkiss @cityofships ideas? comments?