Code review comment for lp:~hazmat/pyjuju/local-respect-series

[1] There are now a few places that precise and oneiric are included in the code. It might be worth it to make these imported constants from a single file CURRENT_UBUNTU_LTS_SERIES="", CURRENT_UBUNTU_SERIES="" and use those imported symbols through the codebase.

[2] You assert juju series in machine unit.
    [2.1] No validation that the series name is valid is preformed, though this might be YAGNI
    [2.2] It seems series is required, we might want to update the docs to reflect that a value must be provided