rbd: use image-spec and snap-spec in rbd help

rbd: use image-spec and snap-spec in rbd help

This patch changes image-name instances to image-spec and snap-name
instances to snap-spec to try to clarify usage for some commands and
disambiguate the term {image,snap}-name, which has been used to denote
both simple names and compound names (specs).

<image-spec> is [<pool-name>]/<image-name>
<snap-spec> is [<pool-name>]/<image-name>@<snap-name>

This patch also removes duplicate checks for image-name and snap-name.

Signed-off-by: Vikhyat Umrao <vumrao@redhat.com>
[idryomov@gmail.com: some commands take either image-spec or snap-spec,
other fixes, formatting, changelog]
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>