Make sure that all nodes always have the correct repos defined

This is a SUSE-only change.
Use --non-interactive with zypper commands
Move duplicated code to a library
Pass --refresh to "zypper addrepo"
Merge pull request #255 from vuntz/enforce-repos

Make sure that all nodes always have the correct repos defined
Revert "enable kernel output on serial console"

This reverts commit 690fad428a59bcf7601a45c3d294cbc733c50deb,
there is already a config option to do that.
Expose serial console attribute via UI

This also changes back the default to "false",
which works around
https://bugzilla.novell.com/show_bug.cgi?id=860088
Make the tty configurable

Depending on your hardware, common serial consoles could be connected
to ttyS0 or ttyS1. Make it configurable in the proposal (hidden
expert setting)