More flexible network access for Launchpad snap builds

[Colin Watson]

Following widespread demand, Launchpad snap builds are now permitted to
access the network (via a proxy) during the "build" phase as well as the
"pull" phase.

On general software engineering principles, it's still good practice to
try to specify your network dependencies in a declarative way so that
they can be fetched systematically and safely during the "pull" phase.
However, for cases where that's prohibitively difficult, this change
should make some categories of snaps easier to build.

Cheers,

-- 
Colin Watson                                       [cjwatson at ubuntu.com]