rhevh7-post: keep files in /var/lib/nfs

Update /etc/rwtab to use files /var/lib/nfs to keep
all files under this dir. Without this patch, the dir
is empty and nfs related services cannot start.

Change-Id: I23772ef9b00db8e69c876d98fcbb7cf41e848175
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1095138
Signed-off-by: Douglas Schilling Landgraf <dougsland@redhat.com>