Fix TRUSTED_NETWORKS parsing (#424)
Fix TRUSTED_NETWORKS parsing (#424)

* Fix TRUSTED_NETWORKS parsing

`''.split(',')` returns `['']`, not `[]`, and `''` doesn't parse as an
IP address range.

* Use nicer parsing suggestion from Bartek

canonical-websites/build.snapcraft.io,canonical-ols/build.snapcraft.io,canonical-ols/build.snapcraft.io,canonical-websites/build.snapcraft.io,canonical-websites/build.snapcraft.io,canonical-ols/build.snapcraft.io