Change metadata-agent to have a configurable backlog

The metadata agent currently runs with a default socket backlog
of 128.  This isn't enough on a busy network node, even when
spawning multiple worker processes.

This change addes a new "metadata_backlog = XX" to the ini file
to support a configurable value to help improve performance.

Change-Id: Ibea398f3b65a56deb1418f39810d87d8360ea9f3
Closes-bug: #1274536
(cherry picked from commit acc21edca49b866e2c2d6e208aaa573d2f95fd88)

Implements: rally-user-story US4049