utils: Make parse_bool a bit more strict

Oly allow definite True values: y, 1, True, true, Yes, yes.
Previously also strings like Yes? would be interpreted as true, but
that's a bit to liberal.

Change-Id: Ie5524550f5444752840f07aac4c3d8d3905750ac
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>