Reorganize the output of lsbackup command

[OmarAyo]

• Added Type of backup (full or incremental) to the output of lsbackup
• Reorganized the output (example below)
• Removed long description as it was too verbose
• Removed unnecessary glog message that prints if the minio bucket exists, this helps with formating the output property (e.g. using jq ). If the bucket doesn't exist will get info about it

Currently the output of lsbackup is:

[Decoder]: Using assembly version of decoder
Page Size: 4096
Listing backups from: /home/omar/Desktop/Dgraph/20.11/bkp
Name	Since	Groups	Encrypted
/home/omar/Desktop/Dgraph/20.11/bkp/dgraph.20210128.144842.388/manifest.json	60005	map[1:[dgraph.type dgraph.graphql.xid dgraph.graphql.schema dgraph.graphql.schema_created_at series score dgraph.graphql.p_sha256hash dgraph.cors dgraph.graphql.p_query dgraph.drop.op dgraph.graphql.schema_history]]	false

This PR will change reorganize it into the following output:

[Decoder]: Using assembly version of decoder
Page Size: 4096
[
	{
		"path": "/home/omar/Desktop/Dgraph/20.11/bkp/dgraph.20210121.125014.852/manifest.json",
		"since": 30005,
		"backup_id": "reverent_vaughan0",
		"backup_num": 1,
		"encrypted": false,
		"type": "full"
	},

Also added verbose flag:

     --verbose           Outputs additional info in backup list.

When enabling it, we will get additional information about groups and DropOperations
e.g.

		"groups": {
			"1": [
				"dgraph.graphql.schema_created_at",
				"dgraph.graphql.xid",
				"dgraph.drop.op",
				"dgraph.type",
				"dgraph.cors",
				"dgraph.graphql.schema_history",
				"score",
				"dgraph.graphql.p_query",
				"dgraph.graphql.schema",
				"dgraph.graphql.p_sha256hash",
				"series"
			]
		}

---

This change is 

[danielmai]

Reviewed 1 of 1 files at r2, 1 of 1 files at r3.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @manishrjain and @vvbalaji-dgraph)