Bypass the database if limit=0 for server-list requests

Not all database engines handle fetching 0 rows the same and there isn't
really a point in going to the database to return nothing, so bypass the
DB query if limit=0.

The Nova API is explicitly checking if limit >= 0 so we can't make any
changes there since they would be backwards incompatible, i.e. to start
enforcing limit > 0.

Closes-Bug: #1296929

Change-Id: Ib3d0318bec972af5f1f25202101e76066f930f34
Merge "Bypass the database if limit=0 for server-list requests"