master_slave parameter is always treated as true 

The master_slave parameter of pacemaker::constraint::colocation seems to be interpreted as true regardless of what it is set to. I believe this is due to evaluating its truthyness in the pcmk_constraint provider instead of comparing it to the symbols :true and :false.

I've created pull request #66 with a proprosed fix for this.

Puppet 4.3.1
