ncm-named: fix retrieval of named root directory from sysconfig

Fixes #241

Wait for more unit tests to be added before merging this PR.
I found why I don't have `bind-chroot` installed... In fact a side effect of the SPMA-YUM migration as `bind-chroot` is not explicitely added anywhere. But I confirm that the end result is that bind is not chrooted anymore. And reinstalling `bind-chroot`, `ROOTDIR` is explicitely set to the default root dir, `/var/named/chroot`.

Thus I'm now convinced by @ned21's remarks! I'll remove the parsing of commented lines and return an empty string (named not chrooted) if the variable `ROOTDIR` is not explicitely defined in `/etc/sysconfig/named`.