Allow specifying the local server with env variable _MINIO_SERVER_LOCAL

[allanrogerr]

Community Contribution License

All community contributions in this pull request are licensed to the project maintainers
under the terms of the Apache 2 license.
By creating this pull request I represent that I have the right to license the
contributions to the project maintainers under the Apache 2 license.

Description

For systems where the hostname cannot be resolved to local IP, this PR introduces a change allowing the specifying of the local server with env variable _MINIO_SERVER_LOCAL

Motivation and Context

In this sort of system, e.g. a kubernetes cluster within which minio pods is running on nodes on different networks, the internal IP address of the pod is not resolvable from any hostname, accessible externally. Therefore minio is not able to recognize itself within the pool of hosts as no endpoint can be resolved to it's own IP address.

A workaround would be to perform the resolution manually by editing /etc/hosts/ and assigning a consecutive host resolving to the pod ip address. e.g.
<ip of k8s pod> <hostname>.<domain>
e.g.
10.42.0.11 minio-18825-1.lab.min.dev

How to test this PR?

See example here: https://github.com/allanrogerr/public/wiki/minio%E2%80%9018825
1.- Pull and compile http://github.com/allanrogerr/minio.git on several k8s pods
2.- Run minio on all pods e.g.

CI=on \
_MINIO_SERVER_LOCAL=minio-18825-1.lab.min.dev:9000 \
~/github/minio/minio server https://minio-18825-{0...1}.lab.min.dev:9000/tmp/data{1...2} --address :9000 --console-address :9090 --certs-dir ~/.minio/certs

3.- Observe the minio cluster online

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• 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)
• Unit tests added/updated
• Internal documentation updated
• Create a documentation update request here