Make add-metric test independent of map ordering.
Allow empty values in juju set-env.
Merge pull request #1020 from tasdomas/fix-1388073

Fix 1388073 and the worker/uniter/jujuc test failure in 1387936

Command line parameters for juju set-env were being parsed incorrectly, not allowing empty values ('key=') - that was fixed and a test added to avoid future regressions.

Tests for worker/uniter/jujuc were failing on PPC due to reliance on map ordering, which is not guaranteed. Fixed.