fix: totalDrives reported in speedTest for multiple-pools

[harshavardhana]

Description

fix: totalDrives reported in speedTest for multiple-pools

Motivation and Context

totalDrives reported in speedTest result were wrong
for multiple pools, this PR fixes this.

Bonus: add support for configurable storage-class, this
allows us to test REDUCED_REDUNDANCY to see further
maximum throughputs across the cluster.

How to test this PR?

Start MinIO server

~ minio server  --address=:9001 /tmp/speedtest{1...4} /tmp/speedtest{5...8}

~ mc admin speedtest myminio-local1/ --duration 20s --size 1MiB --concurrent 4
⠙ Running speedtest (With 1.0 MiB object size, 4 concurrency) PUT: 57 MiB/s GET: 1.0 GiB/s

MinIO 2021-11-27T03:22:40Z, 1 servers, 8 drives
PUT: 57 MiB/s, 57 objs/s
GET: 1.0 GiB/s, 1,029 objs/

Types of changes

• Bug (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)
• Documentation updated
• Unit tests added/updated

[kannappanr]

LGTM

[minio-trusted]

Mint Automation

Test	Result
mint-large-bucket.sh	✔️
mint-fs.sh	✔️
mint-gateway-s3.sh	✔️
mint-erasure.sh	✔️
mint-dist-erasure.sh	✔️
mint-gateway-nas.sh	✔️
mint-compress-encrypt-dist-erasure.sh	✔️
mint-pools.sh	✔️
Deleting image on docker hub
Deleting image locally

[vadmeste]

LGTM