'main_archive_hostname' is not defined

[Andres Rodriguez]

main_archive_hostname and apt_proxy have been removed from the preseed context. As such, old preseeds that had been modified manually, won't; be automatically replaced on upgrade, causing issues like this to arise:

2016-12-02 12:03:06 maasserver: [error] ################################ Exception: name 'main_archive_hostname' is not defined at line 35 column 28 in file
             /etc/maas/preseeds/curtin_userdata ################################

The solution
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The solution should be to fail gracefully and tell the use that the preseed variables are no longer used.