Support parsing of multiseries charms

[bz2]

Fixes lp:1563607 by accepting charms that declare multiple series.
When a charm has a metadata.yaml containing a list of supported
series, the first is taken as the charm's default series.

Charms are always reserialised with a single series. Juju 2.0 is
required for full multiseries charm support.

[axw]

LGTM

[bz2]

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[bz2]

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