Run tests with default concurrency 0

Setting concurrency to 0 would enable Glance tests to be run using the
the total number of cores on the system rather than by a single one.

This would, by default, speed up the run time of tests for developers
who are not aware about it. Also, any jobs which run these tests for
Continuous Integration would be benefitted without having to deal with
the machine level logic.

Closes-Bug: #1368274

Change-Id: I0842b0c0be1272d479fad9ef7609cd8a29231cb5
Merge "Run tests with default concurrency 0"