Revert Juju Version

Pins the Juju agent version to 2.8.1 as the model cleanup can
timeout, but this does not happen in master where the version
is 2.8.1

Bug 1216

Change-Id: I223be2e36cc11ce045aaeffa2c164e592d96ec12
Signed-off-by: beierlm <mark.beierl@canonical.com>