puppetlabs doesnt support the ppc64le arch

[kwmonroe]

I deployed a charm that uses layer-puppet-master on ppc64le, and got the following failure from apt:

W: Failed to fetch http://apt.puppetlabs.com/dists/trusty/Release  Unable to find expected entry 'dependencies/binary-ppc64el/Packages' in Release file

This is because puppetlabs doesn't have any ppc64le packages available (note: this is different from powerpc, which they do have):

http://apt.puppetlabs.com/dists/trusty/Release

The same is true for all the ubuntu series, not just trusty. To work around this, we could check the machine arch in install_puppet_apt_src (https://github.com/jamesbeedy/layer-puppet-agent/blob/master/lib/charms/layer/puppet.py#L117) and simply skip the addition of the constructed apt src on ppc64le. This would cause apt to fall back to the ubuntu archives to install the requested puppet packages.

This workaround would fail, however, if the user requests specific package versions (like v4, which does not exist in the trusty archive, for example). So I'm open to other ideas...