Revert "Remove quota-class subcommand"

This reverts commit 7c11b06bd4e896d891ed3f50c6480bd76a1023bf.

The quota_class subcommand was used to set default quota values
so it shouldn't have been removed.

Related mailing list thread on the topic:
http://lists.openstack.org/pipermail/openstack-dev/2014-May/035383.html

Change-Id: I8f392f8e54bef52c1c950f75377e6bb93a41996d
Partial-Bug: #1299517
Merge "Revert "Remove quota-class subcommand""