lib: drop own asbool implementation and consistently use tg.support.converters as utils2.asbool str2bool never reported error on odd input such as '' or '-1', but the tg asbool behaviour of raising ValueError("String is not true/false: %r" % obj) in that case seems fine.

[tox]
minversion = 1.8
envlist = py27-pytest

[testenv]
setenv =
    PYTHONHASHSEED = 0
deps =
    -r{toxinidir}/dev_requirements.txt
    python-ldap
    python-pam
commands =
    pytest: py.test {posargs}