refs #6317 display instructions before upgrading.

We will by default always run a --dry-run before actually upgrading
and then ask the user for confirmation to execute those updates.
Alternatively a user can use a parameter --yes to avoid asking
for confirmation.
refs #6597 document change re core:update command
Merge pull request #6597 from piwik/6317

Display instructions before upgrading on CLI
refs #6553 fix unit test again. looks like PHPstorm removed a whitespace there
Merge branch 'master' of github.com:piwik/piwik
replace less decimal fields in system tests xml output