External repos in charms - Please consider before relying on them

[Clint Byrum]

Hey everybody. I've noticed a lot of wgets and git clones in charms and
it concerns me a bit.

The idea behind juju is to make it simpler to get on the cloud and to
help people get into a "scale out" mind set. In order to do that, we
need things like 'add-unit' to be reliable.

This means considering how reliable those external services are. So I
have proposed this change to charm store policy:

https://code.launchpad.net/~clint-fewbar/juju/docs-external-repos/+merge/119761

Its really just making an explicit admonition to maintainers to consider
how reliable any external services are in official charms.

Long term I think we'll be able to guarantee reliability of 'add-unit'
by caching downloads and/or booting from snapshotted containers/images.
But that is a long way off, so this policy should help to make sure we
don't get users into a bad situation where they can't scale out when
they need to.