refactor server update behavior

[harshavardhana]

Description

refactor server update behavior

Motivation and Context

add support for minisign signature verification during updates,
this is optional and can also be controlled from air-gapped location.

How to test this PR?

To test this locally build binary locally with this PR

~ GO111MODULE=on CGO_ENABLED=0 go build -tags kqueue -trimpath --ldflags "-s -w -X github.com/minio/minio/cmd.Version=2020-07-21T01:08:20Z -X github.com/minio/minio/cmd.ReleaseTag=DEVELOPMENT.2020-07-21T01-08-20Z -X github.com/minio/minio/cmd.CommitID=da62d7ea0b9829c0ad9fba59a53c47897caff338 -X github.com/minio/minio/cmd.ShortCommitID=da62d7ea0b98 -X github.com/minio/minio/cmd.GOPATH=/home/harsha/mygo -X github.com/minio/minio/cmd.GOROOT=/home/harsha/.gimme/versions/go1.13.13.linux.amd64" -o /home/harsha/mygo/src/github.com/minio/minio/minio 1>/dev/null
~ for i in {02..09}; do cp -a ./minio $(which minio)${i}; done

#!/usr/bin/env bash

export MINIO_PROMETHEUS_AUTH_TYPE=public
export MINIO_ACCESS_KEY="minio"
export MINIO_SECRET_KEY="minio123"
export MINIO_UPDATE_MINISIGN_PUBKEY="RWTx5Zr1tiHQLwG9keckT0c45M3AGeHD6IvimQHpyRywVWGbP1aVSGav"

endpoints=""

for i in {02..09}; do
    endpoints="$endpoints https://localhost:90${i}/tmp/${i}"
done

for i in {02..09}; do
    minio${i} server --certs-dir ../minio-go/testcerts/ --address ":90${i}" ${endpoints} &
done

~ mc admin update myminio-ssl

Would validate the server checksum and signature.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (If yes, please add commit-id or PR # here)
• Documentation needed
• Unit tests needed

[minio-trusted]

Mint Automation

Test	Result
mint-xl.sh	✔️
mint-large-bucket.sh	✔️
mint-fs.sh	✔️
mint-dist-xl.sh	✔️
mint-gateway-s3.sh	✔️
mint-gateway-azure.sh	✔️
mint-zoned.sh	✔️
mint-gateway-nas.sh	✔️
Deleting image on docker hub
Deleting image locally

[nitisht]

LGTM & Tested, minor nit