common/config: intercept "keyfile", translate into "key"

common/config: intercept "keyfile", translate into "key"

The keyfile arg might be - (stdin), which we can only read once.  Ensure
that we consume it once by intercepting the CLI value early and inserting
the value into the 'key' option.

This robs future code of the knowledge that the key came from --keyfile
and not --key, but avoids the issue of multiple users (notably, KeyRing.cc
and the OSD mkfs code).

Remove the - special case from OSD at the same time, since it can no
longer be reached (unless something other than the CLI specified '-', but
neither ceph.conf nor the mon config make sense here).

Signed-off-by: Sage Weil <sage@redhat.com>