metadata-agent: multi-worker support

Neutron Metadata Agent is now able to spawn multiple workers to improve
performances from hundreds or thousands of metadata-namespace-proxy processes.

This change addes a new "metadata_workers = XX" to the ini file
to support creating more workers to help improve performance.

Commit in Icehouse: http://git.io/Al4Xpw

Change-Id: I536dd6a758ec5dbc95a39b485130f8f16a235629
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
Merge "metadata-agent: multi-worker support"