debconf correctly quote strings

Fix https://github.com/ansible/ansible/issues/6819

Tested with:

value='foo2'
value="this shouldn't error"
value='foo2 ; foo3'


Looking at this code I also wonder if https://github.com/ansible/ansible/commit/4e8b97ddeb2fee56cd0a39f7ea1b7eac2c1ba519#diff-f4ae1d864fb6fcbee414ddfa4cfe9a38R355 might have issues double quoting strings?