Dnsdist 1.5 support + updated test framework

[nvaatstra]

Includes:

• xgin's PR 25 to add 1.5 repo support
• Backwards compatibility changes regarding user & group ownership, to handle dnsdist 1.5 default user changes
• Updates to the test setup (newer Ansible versions, newer Python version, updated travis and more recent OS flavors versions

[pieterlexis]

afaik, systemd requires and empty ExecStart to clear the full value. Otherwise it appends. See man systemd.unit

Note that for drop-in files, if one wants to remove entries from a setting that is parsed as a list (and is not a dependency), such as AssertPathExists= (or e.g. ExecStart= in service units), one needs to first clear the list before re-adding all entries except the one that is to be removed. Dependencies (After=, etc.) cannot be reset to an empty list, so dependencies can only be added in drop-ins. If you want to remove dependencies, you have to override the entire unit.

[nvaatstra]

Yep, that's correct and needs to be added back.

It also seems like we need the same thing to be done for the ExecStartPre to avoid that from being triggered multiple times with potentially incorrect parameters.

[nvaatstra]

Both ExecStart & ExecStartPre are now reset if they have an override:

    --- before
    +++ after: /Users/nico/.ansible/tmp/ansible-local-94717z3z152l4/tmpinfnuwf9/override-service.systemd.conf.j2
    @@ -0,0 +1,8 @@
    +[Service]
    +LimitCORE=infinity
    +User=nico
    +Group=nico
    +ExecStart=
    +ExecStart=/usr/bin/dnsdist --supervised --disable-syslog
    +ExecStartPre=
    +ExecStartPre=/usr/bin/dnsdist --check-config