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 agree that this is dangerous and I was not completely satisfied of doing it this way. At the same point, based on our configuration, the default really seems to be `/var/named/chroot` because `named` runs in a chrooted environment. You may want to look at the file contents pased into the unit test that explains the ROOTDIR variable and how it works. In a non chrooted environment, this variable should remain undefined I guess or is just ignored...
I don't remember what's triggering the chroot mode... man page says this is the presence of `bind-chroot` rpm which is not installed... Anyway, in a chroot environment, the configuration file remains `/etc/named.conf` but under the chrooted environment! To update the file actually used you need to know the "chroot prefix" and add it.
The only alternative I see is to hardcode in the component the default ROOTDIR value which is probably `/var/named/chroot`. We should also try to decide whether we want to use ROOTDIR or not depending on whether named is chrooted. I'll be happy to implement it as soon as somebody refresh me on how to detect if named is running chrooted or not.
I wait for more feedback on this and will update the commit based on the discussion/suggestions.