configstore: if the size of the serialised jenv decreases the .jenv file will be corrupt

[Dave Cheney]

Due to dint of fate, or sheer dumb luck, the on disk size of .jenv files always grew -- never shrank.

However, if they do, the new, smaller, contents of the file are written starting from offset 0, _but_not_truncating_ and this causes the previous contents of the file to be visible at the bottom of the file.

The end result is if the size of the environinfo shrinks, possibly because the set of API addresses shrinks, then the jenv file will be corrupted -- readers of the file will see the bit of the yaml which was not overwritten at the end of the file

[Dave Cheney (dave-cheney)]

https://github.com/juju/juju/pull/402#issuecomment-50104078