charm build outputs to trusty if top-level layer doesn't include series

[johnsca]

Checklist

• [X] Confirmed this is an issue with charm-tools, not charmstore-client
• [X] Provide versions of tools used
• [X] Described the feature or ways to replicate the issue

What version am I running?

I ran the following command: charm version and got the following ouput:

charm 2.2.0-0ubuntu1~ubuntu16.04.1~ppa2
charm-tools 2.1.4

I am using: Ubuntu Xenial

Issue/Feature

If a top-level charm layer is depending on a base layer to provide the supported series (e.g., if it's a thin wrapper over the base charm) and thus has no series field in its own metadata.yaml, when you do charm build it doesn't look in the built metadata.yaml for a series, so your multi-series charm ends up in /trusty/.

When Juju 2.0 is officially released, /builds/ should be the default, so that the output dir is always consistent.