Remove quota-class logic from context and make unit tests pass

Continuation of quota-class removal started in
I1110022d6f628d03aaf363da707f2d2ef1600437.

Since we don't have an API to set quota-class any more, we don't need to
store it inside of the context. Also begin removing quota-class logic from
nova.quota to make test_quota pass with changes to context

Change-Id: I85dd24121a434b95dc6f0af180444d4258b50765
Merge "Remove quota-class logic from context and make unit tests pass"