Show non-owner project servers

We can define a role that can list all servers from all tenants via policy like `os_compute_api:servers:detail:get_all_tenants": "is_admin:True or role:int32bit"`. It's very useful to define a readonly admin user, the readonly user can show any server but can't do action on it. In current version, we can list all servers using `--all-tenants`, but we can't show the servers we list. It seems inconsistent and weird. We can add a `--all-tenants` flag as well to `show` subcommand, if the `--all-tenants` flag is present, we will try to find the server from all tenants(if we have permission).