dracut: unset GREP_OPTIONS

GREP_OPTIONS can influence the image creation, if set to user defined
values, so unset it.

https://bugzilla.redhat.com/show_bug.cgi?id=676712

dracut

@@ -278,6 +278,7 @@ fi
 PATH=/sbin:/bin:/usr/sbin:/usr/bin
 export PATH
 unset LD_LIBRARY_PATH
+unset GREP_OPTIONS
 
 [[ $debug ]] && {
     export PS4='${BASH_SOURCE}@${LINENO}(${FUNCNAME[0]}): ';