Add ability to update snapshot name or description

If user forgot to give name and description of the group_snapshot during its creation, then later on if he want to rename and add description of the snapshot then there is no option. It is required because when there is multiple number of snapshot of a group then, it is very difficult to identify the particular snapshot. Moreover, we have the option to create a group from the group_snapshot in which group_snapshot name or UUID is provided. therefore, it is mandatory to have update command for group_snaphsot.