mkcephfs, init-ceph: Warn if hostname "localhost" is seen in ceph.conf.

mkcephfs, init-ceph: Warn if hostname "localhost" is seen in ceph.conf.

Given a ceph.conf that looks like

  [osd.42]
  host = localhost

mkcephfs used to exit with an obscure error message:

  cat: /tmp/mkcephfs.MCBIHvn4Ru/key.*: No such file or directory

"localhost" was never intended to be a valid hostname to use there.
Warn if we see it, and skip the entry. You should use the proper short
hostname of the box.

As init-ceph and mkcephfs share this library, this change affects the
sysvinit scripts too. The behavior *shouldn't* change there (localhost
entries were ignored earlier, too), but you may see this extra
warning. Which is good.

Closes: #3001
Signed-off-by: Tommi Virtanen <tv@inktank.com>