Users may create sites and go above the client's set quota

Any user who may create sites can bypass the quotas set on clients.

To reproduce:

1. Enable quota module
2. Set default quota (eg. 1)
3. Create (or use an existing) client
4. Create a user with the "aegir client" role and associate the client to their user
5. Log-in as the newly created user
6. Create 2 sites

I would normally expect that most users can't bypass the set quotas for a given client.