dns: change default DNS priority of VPNs to -50

Change the default DNS priority of VPNs to -50, to avoid leaking
queries out of full-tunnel VPNs.

This is a change in behavior. In particular:

 - when using dns=default (i.e. no split-dns) before this patch both
   VPN and the local name server were added (in this order) to
   resolv.conf; the result was that depending on resolv.conf options
   and resolver implementation, the name servers were tried in a
   certain manner which does not prevent DNS leaks.
   With this change, only the VPN name server is added to resolv.conf.

 - When using a split-dns plugin (systemd-resolved or dnsmasq), before
   this patch the full-tunnel VPN would get all queries except those
   ending in a local domain, that would instead be directed to the
   local server.
   After this patch, the VPN gets all queries.

To revert to the old behavior, set the DNS priority to 50 in the
connection profile.
23 jobs for bg/dns-vpn in 36 minutes and 53 seconds (queued for 2 minutes and 40 seconds)
latest
Status Job ID Name Coverage
  Test
passed #4930606
checkpatch

00:02:30

manual #4930614
allowed to fail manual
t_centos:7.5.1804
manual #4930615
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t_centos:7.6.1810
manual #4930616
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t_centos:7.7.1908
manual #4930617
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t_centos:7.8.2003
manual #4930618
allowed to fail manual
t_centos:8.1.1911
manual #4930624
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t_debian:10
manual #4930623
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t_debian:9
manual #4930626
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t_debian:sid
manual #4930625
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t_debian:testing
manual #4930607
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t_fedora:28
manual #4930608
allowed to fail manual
t_fedora:29
manual #4930609
allowed to fail manual
t_fedora:30
manual #4930610
allowed to fail manual
t_fedora:31
passed #4930611
t_fedora:32

00:36:53

manual #4930612
allowed to fail manual
t_fedora:33
manual #4930613
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t_fedora:rawhide
manual #4930619
allowed to fail manual
t_ubuntu:16.04
manual #4930620
allowed to fail manual
t_ubuntu:18.04
manual #4930622
allowed to fail manual
t_ubuntu:devel
manual #4930621
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t_ubuntu:rolling
 
  External
failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-code-upstream/1865/

failed https://desktopqe-jenkins.rhev-ci-vms.eng.rdu2.redhat.com/job/beaker-NetworkManager-gitlab-trigger-code-upstream/1865/

05:17:30