Remove check for admin_minion in cephadm_minions

Currently, all minions with the 'admin' role are also required to have the 'cephadm' role. This means you can't have a node which has the ceph.conf file and admin keyring, which isn't also managed by the orchestrator. Can we change this? Because I want my salt master to have the 'admin' role (so I can run `ceph` commands from it), but I _don't_ want the salt master to end up running ceph services (so it needs to not have the 'cephadm' role).

Locate the cephadm_minions role validation and inspect any related tests or configuration checks. The change is done when a node may have the admin role without the cephadm role, allowing a Salt master to run Ceph commands without receiving Ceph services.

Written by the indexing model from the issue text.