[CentOS] lvm snapshot

https://bugzilla.redhat.com/show_bug.cgi?id=1287940

I'm not sure what I saw that made me think the entire drivers/md tree 
was included.  I deleted the first VM that I used to investigate the 
problem, so I can't look at the shell history to figure that one out. 
In any case, that's not what dracut does.

I filed a bug suggesting that dm-snapshot should always be included for 
systems on LVM.