Workers verification for WSGI service

When we start the nova-api service, the *_workers (metadata_workers,
ec2_workers, osapi_compute_workers) value are not verified if the value
is negative. If the user passes a negative value accidentally, and
InvalidInput exception will be threw.

Change-Id: I8337d672cb119efe700aa6be420fdac9864c1577
Closes-bug: #1269997
Merge "Workers verification for WSGI service"