reworked backup and restore templates

I started out by adding @ signs to variables in the templates because I got this deprecation warning:
```
Warning: Variable access via 'dbuser' is deprecated. Use '@dbuser' instead. template[/tmp/vagrant-puppet-3/modules-0/xtrabackup/templates/backupscript.sh.erb]:8
   (at /usr/share/ruby/vendor_ruby/puppet/parser/templatewrapper.rb:76:in `method_missing')
```

Then I also tweaked some minor things in the restore script... and then I _really_ got carried away with the backup script.

IMHO the erb template and resulting shell script both look a lot more readable and clean now.

Functionally I haven't changed a thing, it should all do exactly what it did before.