Fix wrong variable name in the example

Example used `root_db_pass` instead of `mysql_root_db_pass` that is defined.
Check if mysql_db is iterable.
fix condition an if statement range
Merge pull request #19 from pjxiao/master

Check if variable mysql_db is iterable
Merge pull request #17 from shamanime/patch-1

Fix wrong variable name in the example
Merge remote-tracking branch 'upstream/master'