Check if volume service is enabled in quota update

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
Merge "Check if volume service is enabled in quota update"