Tenant <-> realm relationship

[szakhlypa]

The Tenant schema uses the has_one :realm, Realm relationship, but Edgehog allows to create a few realms associated with the same tenant.

test "create_realm/2 allows to create few realms in the same tenant", %{tenant: tenant, cluster: cluster} do
  realm1 = Edgehog.AstarteFixtures.realm_fixture(cluster, name: "realm1")

  valid_attrs = %{name: "realm2", private_key: @valid_private_key}
  assert {:ok, %Realm{} = realm2} = Edgehog.Astarte.create_realm(cluster, valid_attrs)

  tenant = Edgehog.Tenants.preload_astarte_resources_for_tenant(tenant)
  
  # 2 realms are associated with tenant
  assert [realm1, realm2] == Edgehog.Astarte.list_realms()
  # tenant returns only one realm
  assert tenant.realm == Edgehog.Repo.preload(realm1, [:cluster], skip_tenant_id: true) 
end

Also, if there are multiple realms associated with the same tenant, the reconciliation and cleanup after delete tasks will only use one realm.