snapshot_create_as: Add test case for "snapshot=no" in diskspec option

When snapshot=no given in diskspec, the specific disk will not be taken
snapshot. For support this, check "snapshot=no" in compose_disk_options
function and update cfg file.

Signed-off-by: Wayne Sun <gsun@redhat.com>
Merge pull request #168 from waynesun09/diskspec_snapshot_no

snapshot_create_as: Add test case for "snapshot=no" in diskspec option