Add option so mysql::backup to dump each database to its own file

I added a **file_per_database** parameter to mysql::backup that will modify the mysqlbackup.sh script to loop over all databases and dump each database to it's own file.

Alternatively, a "magic" value for backupdatabases could be used that performs the same changes as file_per_database, something like ```backupdatabases => 'each' ```.  

Also added rspec-system based tests for the mysql::backup class.