backup and snapshot commands to be renamed for volume resource

backup and snapshot commands to be renamed for volume resource

As backup and snapshots are directly related to volume resource, these must be renamed to
volume backup and volume snapshot to depict resource relation. This will be helpful for user to generalized the way things are searched.

snapshot create [-h] [-f {json,shell,table,value,yaml}] [-c COLUMN]
                       [--max-width <integer>] [--noindent] [--prefix PREFIX]
                       [--name <name>] [--description <description>] [--force]
                       <volume>

should be

volume snapshot create [-h] [-f {json,shell,table,value,yaml}] [-c COLUMN]
                       [--max-width <integer>] [--noindent] [--prefix PREFIX]
                       [--name <name>] [--description <description>] [--force]
                       <volume>

and

backup create [-h] [-f {json,shell,table,value,yaml}] [-c COLUMN]
                     [--max-width <integer>] [--noindent] [--prefix PREFIX]
                     [--name <name>] [--description <description>]
                     [--container <container>]
                     <volume>
should be

volume backup create [-h] [-f {json,shell,table,value,yaml}] [-c COLUMN]
                     [--max-width <integer>] [--noindent] [--prefix PREFIX]
                     [--name <name>] [--description <description>]
                     [--container <container>]
                     <volume>

The same way other commands for backup and snapshot must be prefixed with volume.
Here is the list for proposed commands:
  snapshot delete
  snapshot list
  snapshot set
  snapshot show
  snapshot unset
and
  backup delete
  backup list
  backup restore
  backup show