Add quotes around true/false.

To make sure they're YAML strings.  Otherwise, the template creation
will fail with:

heat stack-create -f https://raw.githubusercontent.com/rackspace-orchestration-templates/minecraft/master/minecraft-server.yaml -P ssh_keypair_name=jasond1 -P minecraft_spawn_animals=true m1
ERROR: "true" is not an allowed value [True, False]