install_sources should adapt automatically to the Ubuntu series

[Andrea Ieri]

install_sources currently defaults to a Xenial repo, but the charm should be able to determine which series it's running on automatically.

It may be useful to retain the config option to satisfy corner cases, but I don't think operators should generally be required to configure install_sources whenever they're not deploying on Xenial.

[George Kraft (cynerva)]

Following the trail[1][2][3] from azure-integrator's config.yaml, it looks like this could be a one-line change from 'xenial' to '{series}' in the install_sources default value. Would need testing.

[1]: https://github.com/charmed-kubernetes/charm-azure-integrator/blob/d184ad25cf316145f99fb43a0947e0f049c5455b/config.yaml#L54-L56
[2]: https://bugs.launchpad.net/layer-apt/+bug/1796993
[3]: https://github.com/juju/charm-helpers/issues/229