Unify log/data dir paths into one package

Changed all uses of DataDir and LogDir to use a function that
generates those paths.
This change prevents the paths to be generated always which breaks
juju client in windows when series is unknown (which happens with
some window versions we still did not discover) because it will
try to check if the directory exists (and by default it will not
on windows clients)
I don't think this is right. DataDir/LogDir should be based on the OS of the machine being started, not the current machine.