lower layers that define series will be included in built charm

[battlemidget]

Otherwise, if you have a feature request or issue with any other charm command please delete the above and continue below

Checklist

• Confirmed this is an issue with charm-tools, not charmstore-client
• [X ] Provide versions of tools used
• 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 2.1.1-0ubuntu1
charm-tools 2.1.2

I am using: ubuntu

Issue/Feature

Doing a charm build that includes layers such as nginx which define a series in its metadata.yaml will be included in the top level charm being built. This results in the finished charm including either multiple series name (ie: series: ['xenial', 'trusty', 'xenial']) or set the final charm up to support additional series that may not have been developed for.

I expect/expected the following

If this toplevel contains a series definition it should use that as the authoritative source of what series it supports.

[ktsakalozos]

Happens here as well. This issue might be related: #186