allow passing array of strings to waitforneedle

simplifies api as test doesn't have to call needle::tags and assemble
the array itself
add workaround for yast popup in second stage (bnc#836612)
inroduce dents

If a test has passed with minor defects that is not fatal for a
milestone the test can have a 'dent'. This is useful for e.g. popups
that appear in yast when some minor error occurs that does not disturb
the overall success of the installation.