Proper selinux logic for Horizon

According to here:
http://docs.puppetlabs.com/learning/variables.html
"all facts are strings, and all non-empty strings,
 including the string "false" . are true"

Checking "if $::selinux" will always return true,
just like (if 'hello':) in python will always return true.

This uses str2bool to ensure that the variable is evaluated as a boolean.

Change-Id: I9489c3bd9a217c80762f5875f026676268572d79
(cherry picked from commit ac5be9605f4801996a20c5e86b796fcb65dacb0b)