Quotas fail to load on the project quota update form in the
admin dashboard when the volume service is disabled.
Change the get_default_quota_data and get_tenant_quota_data functions
to always check for a disabled volume service, so we don't have to
remember to do it every time we call the functions.
Moved *QUOTA_FIELDS to usage/quotas.py to avoid a circular import.
Fixes bug #1155878.
Change-Id: I7150c4d7c318b6199ff03825cf5d770c7741b379
1935a3d8d2