Fix bug 564

This fixes bug 684, which noted that the workload message was not being
passed to the callback when removing charms.

This also fixes some lint errors and adds tear-down of the per-ns model
in integration tests

Change-Id: I83be9e3d951cf10a7479ea2e55074526403cf674
Signed-off-by: Adam Israel <adam.israel@canonical.com>