NetworkService: changed API to support more backends (namely wicked)

The change is backward incompatible
Updated changelog
Replace runlevel with default target in public API
Merge pull request #137 from vmoravec/obsolete-runlevel

Replace runlevel with systemd default target in public API
remove log suppressing attribute, used only on one place and looks useless
Merge pull request #138 from jsrain/master

added dedicated "Show Release Notes" button support for NCurses
Replace runlevel entries with default_target in test output
Merge pull request #139 from vmoravec/fix-tests

Fix broken tests which relied on runlevel entries in output; replaced by default_target.
Merge pull request #140 from jsrain/master

fixed date in changelog
rewrote the last minitest to rspec (gh#yast/yast-rake#2)

Partial fix of `rake test:unit`
https://github.com/yast/yast-rake/issues/2
fix running tests from another directory (like `rake test:unit`)
Merge pull request #142 from mvidner/fix-rake-test

Fix `rake test:unit`
remove dead code
remove interface only for testing
remove unused publish for private methods
remove publish of unused methods
simplify hotplug regexp
remove more dead code
DRY hotplug devices matching

There is difference in "pci" value. I don't see any reason why to have
it different, so there is definitivelly in one of case bug. Let's see
who complain about it and fix it so it works in all cases.
remove usused methods