Support placeholder for series in sources

[Cory Johns]

If a charm that supports multiple series has an install source that requires the series be specified, like "deb https://packages.microsoft.com/repos/azure-cli/ xenial main", there's currently no way for the charm to update that series if that source is specified as the default value for the install_sources config.

I would be nice if the layer supported saying something like "deb https://packages.microsoft.com/repos/azure-cli/ {series} main" where "{series}" would be automatically replaced with the correct value.

I know that this config data is currently just passed on to charmhelpers, so maybe this would be better done there, but I wanted to know if it seems reasonable and if it would be better to do in the layer as a pre-process step vs. in charmhelpers?

[Stuart Bishop (stub)]

It seems reasonable. I don't think we need other variables than 'series', at least to start with.

I think doing this in charm-helpers is preferable, unless there is some reason it is easier to create or maintain in the apt layer. charm-helpers also has a test suite.

[Cory Johns (johnsca)]

Moved to https://github.com/juju/charm-helpers/issues/229