Code review comment for lp:~stub/charm-helpers/fix-configure_sources

That is a great addition to the mechanism IMO.

Could you please add a short docstring to the function explaining what the key parameter should be like? Example:

"""This function adds an entry to the sources.list file.

@param source: The archive's URL. Example: "http://archive.ubuntu.com"
@param key: The GPG key to verify the archive packages with. Can be either a
    key ID or a full public key block. If only the key ID is specified the
    key will be fetched from the network via HKP."""