Comment 8 for bug 1402055

I'm done with the changes for the review and please review the changes.

Now trove-taskmanager service is able to startup without nova_compute_url and we don't need to be disable exists_notification_transformer as workaround.

Since we are using nova admin credentials with valid trove_auth_url, 'trove.extensions.mgmt.instances.models.NovaNotificationTransformer' can make use of trove.common.single_tenant_remote module to get the nova admin client.