assess_heterogeneous_control does not honour series

As seen at
    http://reports.vapour.ws/releases/issue/576aab41749a5627259844f7

While we can pass --series xenial to bootstrap xenial, the default-series from the config is used. the charm series checks will never pass if the env sets a different series from the script.

We need to pass the series down the envs and be clear about which series charms should be.

UPDATE
The issue is entangled with the charm. trusty was the default series for the dummy-* charms. The script deployed the charm and got trusty though the intent was to test xenial.
The dummy charms were updated to set xenial as the default, now the test fails when
trusty is passed to the script. Xenial works.