configuration parsing: give better error for missing =

A ceph.conf line with "key" and no "= value" currently shows
"unexpected character while parsing putative key value,
at char N line M".  There's no reason it can't be clearer.

Fixes: #4229
Signed-off-by: Dan Mick <dan.mick@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>