Change metadata-agent to spawn multiple workers

There is currently only one metadata-agent per network node,
which could be handling connections 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.

Partial-bug: #1274536
Implements: rally-user-story US4047
Implements: rally-task TA3195
Upstream-Review: https://review.openstack.org/#/c/70204/
Not-in-upstream: false