new alpha beta support broke CI

[Ian Booth]

> PS. I still don't know the arch name for PPC64el tools. CI is
> createing ppc64el and ppc64 each time it sees the former. My
> deployment to stilson-06 implies ppc63el is ignored, the arch has to
> be ppc64
> 

The expected arch name for ppc64el tools is "ppc64". So the tarball naming will
be something like:

juju-1.18.4-trusty-ppc64.tgz