nfs-utils: Install nfsmount.conf

The file nfsmount.conf is present in nfs-utils but not installed to the
filesystem.  It has been added to nfs-utils-client as it seemed to be
missing accidentally rather than being removed intentionally.  The binary
files in nfs-utils-client have a hard-coded reference to it and we
already provide the manpage for it in nfs-utils-doc.

Signed-off by: Rob Woolley <rob.woolley@windriver.com>