Parameterize the quota default values

This allows site operators to increase the default values for tenants.
This is applicable even if the site operator uses the DbQuotaDriver
since if a specific overriden value does not exist for the tenant,
the default value from the quantum.conf.erb will be enforced.

Partial-bug: #1228623

Change-Id: Ibcde2497ddd4954cbc7f2030fb1afd4d6d49b442