tcollector.py: refactor the way adding default tags works, so for any line,
it will add the tags if they're not already present.
This allows for defaults that can be overridden in a collector.

Signed-off-by: Vasiliy Kiryanov <vasiliy.kiryanov@gmail.com>
iostat.py: add statistic metric svctm.
Signed-off-by: Vasiliy Kiryanov <vasiliy.kiryanov@gmail.com>
Merge remote-tracking branch 'upstream/master' into remerge-upstream