PE cumulus fixes

This PR updates the pe_ship.rake file's ship_debs task to not assume an
all-arch package should be shipped to every target, but to handle the case of
cumulus, for which we only ship powerpc. It also updates the reprepro shipping
to add to repos separately for each dist, because we may be shipping cumulus,
which goes into different repositories.

In the pbuilderrc we lay down in the Cumulus builder node definitions, we pass
a URL that is partially composed of an environment variable, 'NETWORK_OS.' We
need to set this environment variable in order to create the url correctly.
This PR updates it for the first in what will likely be more than a few
required NETWORK_OS variations, i.e. cumulus.

This PR also updates both the legacy jenkins task and the current dynamic task
workflow to support building on cumulus by checking for a cumulus cow and then
building on a node with that label.