Use string comparison of facter booleans

I accidentally removed this when doing the puppet-lint cleanup, since
puppet-lint considers testing for a boolean value with quotes an error.
Use stdlib::str2bool to test facter booleans

An issue filed against puppet-lint [1] recommends to use str2bool
instead of testing quoted facter values. This is probably slightly
better since we can avoid disabling the check entirely in puppet-lint.

Apparently Facter 2 is supposed to fix this? Not sure why it isn't,
should maybe look into it.

[1] https://github.com/rodjek/puppet-lint/issues/251