peergrouper tests badly need fixing

[Tim Penhey]

Hi all,

Using Go 1.5 on trusty has the peergrouper tests failing very often. 1
or 2 almost every time.

Dave suggested trying "GOMAXPROCS=1" which will make the scheduler look
more the like 1.2.1 scheduler. This has the tests pass.

Obviously we have timing related issues here.

Bug filed:
  https://bugs.launchpad.net/juju-core/+bug/1496237

And added to the tech-debt kanban board.

Tim