Use "str2bool_i" instead of "== 'true'"

Not tested yet, we should test and merge when convenient. If the code changes so that new `== true` appear, this commit can be easily regenerated by running this oneliner in Astapor's root dir:

    grep -ril "== 'true'" puppet | xargs sed -i -e "s/map_params('\([^']*\)') == 'true'/str2bool_i(map_params('\1'))/"

and then verifying that this doesn't print anything:

    grep -ri "== 'true'" puppet