Return Customer's Quota Usage through Admin API

DocImpact: This patch adds an extension 'UsedLimitsForAdmin'. If this
extention is enabled then it extends the used limits API behavior, such
that admin can fetch the details of any customer's quota usage by passing
the customer's tenant id in query parameters.The API signature for the
same is 'v2/{tenant_id}/limits?tenant_id={customer_tenant_id}'

Change-Id: I89b8b5083e46b899458407426c89a3865e960faa
Implements: blueprint customer-quota-through-admin-api
Merge "Return Customer's Quota Usage through Admin API"