ceph: use "config set" consistently in help/error msgs

apparently it was once known as set_config.  Fix up everything to
refer to the new name.  Also, fix up the help message.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
osd: fix name of setomapval admin-daemon command

Signed-off-by: Dan Mick <dan.mick@inktank.com>
ceph: ceph mon delete doesn't exist; ceph mon remove is the command
Fix up cli test as well (doc is already correct)

Signed-off-by: Dan Mick <dan.mick@inktank.com>
mds: error messages for export_dir said 'migrate_dir'

Signed-off-by: Dan Mick <dan.mick@inktank.com>
ceph: fix 'pg' error message to direct user toward better input

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Merge branch 'wip-cephtool' into next

Usage/errmsg fixups for the ceph CLI tool

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>