update CentOS build instructions
Merge branch 'master' of https://github.com/rsyslog/rsyslog
doc: update build requirements
remove no longer needed variable
doc: finalize package requirements for openSUSE
doc: add setup instructions for mysql testbench
undo commit 077d59c2c7984eb0273be949969585be241408c8 due to regression

There is a regression in this feature-add commit and I don't have the
time to search for it. Refering it back to contributor.

see also https://github.com/rsyslog/rsyslog/pull/145
Merge branch 'master' of git+ssh://git.adiscon.com/git/rsyslog