Added new parameters for Neutron agents

Upstream fixes for Neutron introduced two new configuration
parameters: 'metadata_workers' and 'metadata_backlog'.

We need to setup these parameters to production oriented values.

Change-Id: I2a23bd75c9acfeebb82d6921874dec33eb0b192b
Closes-Bug: #1336810
Merge "Added new parameters for Neutron agents"