mgr/cephadm: CephadmService.create -> CephadmService.prepare_create

mgr/cephadm: CephadmService.create -> CephadmService.prepare_create

Refactored this to simplify the control flow. It became obvious that

* `CephadmService.make_daemon_spec`
* `CephadmService.prepare_create`
* `CephadmService.generate_config`

are basically the same thing. They're all called directly
before the deployment of a daemon. All of them should be
unified. This PR makes this refactorization simpler

By renaming `create` to `prepare_create`, we make `create`
no longer being the entrypoint to call
`create_daemon`. Thus all the functions above
return some data structures.

Signed-off-by: Sebastian Wagner <sebastian.wagner@suse.com>
(cherry picked from commit 245cf50132e1731982d19e1114116e4150b9ea97)